<PAGE>

                                                                     EXHIBIT 4.3
                              CAIS INTERNET, INC.

                            SERIES G PREFERRED STOCK
                               PURCHASE AGREEMENT

     This SERIES G PREFERRED STOCK PURCHASE AGREEMENT (this "Agreement"), dated
                                                             ---------
as of March 20, 2000, by and between CAIS Internet, Inc., a Delaware corporation
(the "Company"), and 3Com Corporation, a Delaware corporation (the "Purchaser").
      -------                                                       ---------

                                   Section 1

                   Authorization and Sale of Preferred Stock
                   -----------------------------------------

     1.1   Authorization.  The Company has authorized the sale and issuance of
           --------------
20,000 shares of the Company's Series G Preferred Stock, par value $.01 per
share ("Shares").
        ------



     1.2   Sale and Issuance of Shares.  Subject to the terms and conditions of
           ---------------------------
this Agreement, Purchaser agrees to purchase from the Company, and the Company
agrees to sell and issue to Purchaser, the Shares at a purchase price of One
Thousand Dollars ($1,000.00) per Share.

                                   Section 2

                            Closing Dates; Delivery
                            -----------------------

     2.1   Closing.  The purchase and sale of the Shares hereunder shall take
           -------
place at a closing (the "Closing") on the third business day after the
                         -------
satisfaction or waiver of the conditions set forth in Sections 5 and 6 hereof
(the "Closing Date").  The Closing shall be held at the offices of the Company
      ------------
at 1255 22nd Street, N.W., Washington, D.C., at 10:00 a.m. local time, on the
Closing Date, or at such other time and place upon which the Company and
Purchaser shall agree.

     2.2   Delivery.  At the Closing, the Company will deliver to Purchaser a
           --------
certificate registered in Purchaser's name representing the Shares to be
purchased against payment of the purchase price therefor.  At the Closing,
Purchaser will pay to the Company by wire transfer of immediately available
federal funds per the Company's instructions cash in the amount of Twenty
Million Dollars ($20,000,000.00).

                                   Section 3

                 Representations and Warranties of the Company
                 ---------------------------------------------

     The Company (including for purposes of this Section 3 all subsidiaries of
the Company) represents and warrants to Purchaser as of the date of this
Agreement as follows:

     3.1   Organization; Good Standing.    The Company is a corporation duly
           ---------------------------
organized, validly existing and in good standing under the laws of the State of
Delaware, is qualified to conduct business and is in good standing under the
laws of each jurisdiction in which the nature of its business or the ownership
or leasing of its properties requires such qualification.  The Company has the
requisite corporate power and authority to own and operate its properties and
<PAGE>

assets and to carry on its business as currently conducted and as proposed to be
conducted.  A true and correct copy of the Company's Amended and Restated
Certificate of Incorporation (the "Restated Certificate") is attached hereto as
                                   --------------------
Exhibit A, and a true, correct and complete copy of the Company's By-Laws (the
---------
"By-Laws") is attached hereto as Exhibit B, which in each case shall be in full
--------                         ---------
force and effect as of the Closing Date (as the Restated Certificate shall be
amended pursuant to the Certificate of Designation (as defined below)).

     3.2   Company Shares.  The Shares to be issued to Purchaser hereunder will
           --------------
be duly authorized, validly issued, fully paid and nonassessable, and will have
the rights, preferences and privileges described in the Restated Certificate and
the Certificate of Designation of Series and Determination of Rights and
Preferences of Series G Convertible Preferred Stock (the "Certificate of
                                                          --------------
Designation"), a true and correct copy of which is attached hereto as Exhibit C,
-----------                                                           ---------
which shall be in full force and effect as the Closing Date.  Such copy contains
all amendments through the Closing Date.  The Company will not amend the
Certificate of Designation or otherwise take any action or fail to take any
action that would cause or permit the Certificate of Designation to be amended
or adversely affect the rights, preferences and privileges of the Shares without
Purchaser's prior written consent.

     3.3   Authority; Binding Nature of Agreements.
           ---------------------------------------

     (a)   The execution, delivery and performance of this Agreement and all
other agreements and instruments contemplated to be executed and delivered by
the Company in connection herewith have been duly authorized by all necessary
corporate action on the part of the Company and its board of directors (the
Board").  The Company has all requisite legal and corporate power and authority
------
to sell and issue the Shares hereunder and to issue the shares of common stock
of the Company, par value $0.01 per share, issuable upon conversion of the
Shares (the "Conversion Stock") and to carry out and perform its obligations
             ----------------
under the terms of this Agreement.

     (b)   The Conversion Stock has been duly and validly reserved and, when
issued in compliance with the terms of this Agreement and the Restated
Certificate, will be validly issued, fully paid and nonassessable, and will have
the rights, preferences and privileges described in the Restated Certificate and
the Certificate of Designation.  The Shares will be delivered to Purchaser free
of any liens or encumbrances, other than any liens or encumbrances created by or
imposed upon or by virtue of Purchaser; provided, however, that the Shares and
                                        --------  -------
the Conversion Stock are subject to restrictions on transfer under state and/or
federal securities laws as set forth herein and in the Registration Rights
Agreement between the Company and Purchaser dated the date hereof (the
"Registration Rights Agreement").  Except as set forth in the Restated
------------------------------
Certificate and the Certificate of Designation, the Shares and the Conversion
Stock are not subject to any preemptive rights or rights of first refusal.

     (c)   This Agreement and all other agreements and instruments contemplated
to be executed and delivered by the Company in connection herewith constitute
the legal, valid and binding obligations of the Company, enforceable against the
Company in accordance with their terms, except to the extent that enforceability
may be limited by applicable bankruptcy, merger, insolvency, moratorium,
fraudulent conveyance or other laws affecting the enforcement of the rights of
creditors and other obligees generally and by general principles of equity
regardless of whether such enforceability is considered in a proceeding in law
or equity, and by limitations on indemnification due to public policy
considerations.

                                       2
<PAGE>

     3.4   No Violation or Default.
           -----------------------

     (a)   The execution, delivery and performance of this Agreement and the
other agreements and instruments contemplated to be executed and delivered by
the Company in connection herewith have not and will not, directly or indirectly
(with or without notice or lapse of time):

          (i)    contravene, conflict with or result in a material violation of
(i) the Company's Restated Certificate or Bylaws, or (ii) any resolution adopted
by the Company's Board or any committee thereof or the stockholders of the
Company;

          (ii)   contravene, conflict with or result in a violation of, or give
any governmental body the right to challenge the issuance of the Shares or to
exercise any remedy or obtain any relief under, any legal requirement or any
order to which the Company or any material assets owned or used by it are
subject;

          (iii)  to the knowledge of the Company, cause any material assets
owned or used by the Company to be reassessed or revalued by any taxing
authority or other governmental body;

          (iv)   contravene, conflict with or result in a violation of any of
the terms or requirements of, or give any governmental body the right to revoke,
withdraw, suspend, cancel, terminate or modify, any governmental authorization
that is held by the Company;

          (v)    contravene, conflict with or result in a violation or breach
of, or default under, any material contract to which the Company is a party; or

          (vi)   result in the imposition or creation of any mortgage, pledge,
lien, charge or encumbrance upon or with respect to any material properties or
assets owned or used by the Company.

     (b)   The Company is not in violation of or in default under its Restated
Certificate or By-Laws or, to the best of the Company's knowledge, (i) any
judgment, order, writ, decree, statute, rule or regulation applicable to it, or
(ii) any mortgage or indenture, or any other material agreement, instrument or
contract to which it is a party or by which it is bound.

     3.5   Finders and Brokers.
           -------------------

     The Company has not engaged any broker, finder or agent, and the Purchaser
has not, and will not, incur, directly or indirectly, as a result of any action
taken by the Company, any liability for brokerage or finders' fees or agents'
commissions or any similar charges in connection with the Transaction Documents
or the transactions contemplated hereby.  In the event that the preceding
sentence is in any way inaccurate, the Company agrees to indemnify and hold
harmless Purchaser from any liability for any commission or compensation in the
nature of a finder's fee (and the costs and expenses of defending against such
liability) for which Purchaser, or any of its officers, directors, employees or
representatives, is responsible.

                                       3
<PAGE>

     3.6   Reports and Financial Statements; Absence of Certain Changes.
           ------------------------------------------------------------

     (a)   The Company has filed all reports required to be filed with the U.S.
Securities and Exchange Commission ("SEC") pursuant to the Securities Act of
                                     ---
1933, as amended (the "Securities Act"), and the Securities Exchange Act of 1934
                       --------------
as amended (the "Exchange Act"), since its initial public offering on May 20,
                 ------------
1999 (all such reports, including those to be filed prior to the Closing Date
and all registration statements and prospectuses filed by the Company with the
SEC in connection with the Company's initial public offering, are collectively
referred to as the "Company SEC Reports"), and has previously furnished or made
                    -------------------
available to Purchaser true and complete copies of all the Company SEC Reports
filed, if any, with respect to periods ending after May 20, 1999 (including any
exhibits thereto) and will promptly deliver to Purchaser any Company SEC Reports
filed between the date hereof and the Closing Date.  All of such Company SEC
Reports complied at the time they were filed and declared effective, if
applicable, in all material respects with applicable requirements of the
Securities Act and the Exchange Act and the rules and regulations thereunder.
None of such Company SEC Reports, as of their respective dates (as amended
through the date hereof), contained any untrue statement of a material fact or
omitted to state a material fact required to be stated therein or necessary to
make the statements therein, in light of the circumstances under which they were
made, not misleading.  The audited financial statements of the Company included
in the Company SEC Reports comply in all material respects with the published
rules and regulations of the SEC with respect thereto, and such audited
financial statements (i) were prepared from the books and records of the
Company, (ii) were prepared in accordance with United States generally accepted
accounting principles ("GAAP") applied on a consistent basis (except as may be
                        ----
indicated therein or in the notes or schedules thereto) and (iii) present fairly
the financial position of the Company as of the dates thereof and the results of
operations and cash flows for the periods then ended.  The unaudited financial
statements included in the Company SEC Reports comply in all material respects
with the published rules and regulations of the SEC with respect thereto; and
such unaudited financial statements (i) were prepared from the books and records
of the Company, (ii) were prepared in accordance with GAAP, except as otherwise
permitted under the Exchange Act and the rules and regulations thereunder, on a
consistent basis (except as may be indicated therein or in the notes or
schedules thereto) and (iii) present fairly the financial position of the
Company as of the dates thereof and the results of operations and cash flows (or
changes in financial condition) for the periods then ended, subject to normal
year-end adjustments and any other adjustments described therein or in the notes
or schedules thereto.  The foregoing representations and warranties shall also
be deemed to be made with respect to all filings made with the SEC on or before
the Closing Date.

     (b)   Except as specifically contemplated by this Agreement or reflected in
the Company SEC Reports, since May 20, 1999, there has not been (i) any material
adverse change in the Company's business, assets, liabilities, or operations,
and, to the knowledge of the Company, no event has occurred that is likely to
have a material adverse effect on the Company's business, assets, liabilities or
operations; or (ii) any material change in the Company's accounting principles,
procedures or methods.

     3.7  Compliance with Applicable Law.  Except as disclosed in the Company
          ------------------------------
SEC Reports filed prior to the date of this Agreement or in the Company's
Registration Statement on Form S-1, as amended, SEC file number 333-72769
("Registration Statement"), the Company holds all governmental authorizations
------------------------
necessary for the lawful conduct of its business under and

                                       4
<PAGE>

pursuant to, and the business of the Company is not being conducted in violation
of, any governmental authorization applicable to the Company, except to the
extent that the failure or violation would not in the aggregate have a material
adverse effect on the business, results of operations or financial condition of
the Company.


     3.8   Consents.  No consent, approval or authorization of or designation,
           --------
declaration or filing with any governmental authority or other third party on
the part of the Company is required in connection with the valid execution and
delivery of this Agreement nor any other agreement or instrument contemplated to
be executed and delivered by the Company in connection with this Agreement, or
the offer, sale or issuance of the Shares and the Conversion Stock, or the
consummation of any other transaction contemplated hereby or thereby, except for
(a) qualification (or taking such action as may be necessary to secure an
exemption from qualification, if available) of the offer and sale of the Shares
and the Conversion Stock under applicable blue sky laws, which qualifications,
if required, will be accomplished in a timely manner, (b) filings or notices
required under the Hart-Scott-Rodino Antitrust Improvements Act of 1976, as
amended ("HSR Act"), and (c) such other consents, approvals, authorizations and
filings as have been obtained and are in full force and effect.


     3.9   Capitalization.  As of the date of this Agreement, the authorized
           --------------
capital stock of the Company consists of 100,000,000 shares of Common Stock, of
which 23,119,061 shares are issued and outstanding, and 25,000,000 shares of
preferred stock, par value $.01 per share, of which 2,827,168 shares are
designated as Series A Preferred Stock (the "Series A Shares"), none of which
                                             ---------------
are issued and outstanding, 1,119,679 shares are designated as Series B
Preferred Stock, none of which are issued and outstanding, 125,000 shares are
designated as Series C Preferred Stock ("Series C Shares"), of which 125,000
shares are issued and outstanding, 9,620,393 shares are designated as Series D
Convertible Participating Preferred Stock ("Series D Shares"), 5,276,622 of
which are issued and outstanding, 9,620,393 shares are designated as Series E
Convertible Participating Preferred Stock ("Series E Shares"), none of which are
issued and outstanding, 56,617 shares are designated as Series F Convertible
Preferred Stock ("Series F Shares"), none of which are issued and outstanding,
and 28,051 shares are designated as Series G Convertible Preferred Stock
("Series G Shares").  Immediately prior to Closing, no shares of Series G
Preferred were issued and outstanding.  The outstanding shares of Common Stock
have been duly authorized and validly issued in compliance with applicable laws,
and are fully paid and non assessable.  As of the date of this Agreement, the
Company has reserved (a) 779,195 shares of Common Stock for issuance upon
conversion of the Shares to be issued hereunder or upon additional shares issued
as dividends on the shares issued hereunder, (b) 1,250,000 shares of Common
Stock for issuance upon conversion of the Series C Shares, (c) 5,000,000 shares
of Common Stock for issuance pursuant to the Company's Amended and Restated 1998
Equity Incentive Plan and Atcom, Inc. 1996 Stock Option/Stock Issuance Plan, of
which 711,475 shares have been issued pursuant to the exercise of granted
options and options to purchase 2,172,643 shares are outstanding, (d) 4,809,061
shares of Common Stock for issuance pursuant to other outstanding options and
warrants, of which 18,000 shares have been issued pursuant to the exercise of
granted options or warrants, (e) 14,896,216 shares of Common Stock for issuance
upon conversion of the Series D Shares and Series E Shares, and (e) 1,572,677
shares of Common Stock for issuance upon conversion of the Series F Shares.
Except as set forth above and as provided in the Company's Restated Certificate
or Certificate of Designation, there are no

                                       5
<PAGE>

other options, warrants or other rights to purchase or acquire any of the
Company's authorized and unissued capital stock.

     3.10  Environmental Matters.  The properties, assets and operations of
           ---------------------
the Company are in material compliance with all applicable federal, state, local
or foreign laws, rules, regulations, permits, licenses and decrees relating to
environmental matters or the discharge, release, storage, treatment or clean-up
of any materials or substances.  No environmental or similar claim has been
asserted (or to the best knowledge of the Company threatened) against the
Company.


     3.11  Litigation.  There are no actions, suits, proceedings or
           ----------
investigations pending against the Company or its properties before any court or
governmental agency which could reasonably be expected to have a material
adverse effect on the business, results of operations or financial condition of
the Company (nor, to the best of the Company's knowledge, is there any threat
thereof).  The Company is not a party or subject to any order, writ, injunction
or decree of any court or governmental agency or instrumentality.  There is no
action, suit, proceeding or investigation by the Company currently pending or
that the Company intends to initiate.

     3.12  Title to Properties and Assets; Liens.  The Company has good and
           -------------------------------------
marketable title to its properties and assets, and has good title to all its
leasehold interests, in each case subject to no mortgage, pledge, lien, lease,
encumbrance or charge, other than (i) the lien of current taxes not yet due and
payable, (ii) possible minor liens and encumbrances which do not in any case
materially detract from the value of the property subject thereto or materially
impair the operations of the Company, and which have not arisen otherwise than
in the ordinary course of business and (iii) liens to secure vendor financing or
installation purchases.  Such properties and assets are sufficient to enable the
Company to carry on its business as presently conducted and, to the Company's
knowledge, as proposed to be conducted.

     3.13  Intellectual Property; Trademarks.
           ---------------------------------

           (a)  Except as set forth on Schedule 3.13, the Company owns or has
                                       -------------
the right to use, free and clear of all liens, charges, claims and restrictions,
all patents, trademarks, service marks, trade names, copyrights, licenses and
other intellectual property rights necessary to the business of the Company as
presently conducted. Except as set forth on Schedule 3.13, there are no
                                             -------------
outstanding options, licenses, or similar agreements relating to the foregoing,
nor is the Company bound by or a party to any options, licenses or agreements of
any kind with respect to the patents, trademarks, service marks, trade names,
copyrights, trade secrets, licenses, information, proprietary rights and
processes of any other person or entity, in each case, other than end-user
licenses and marketing, distribution and reselling agreements entered into in
the ordinary course of business.  To the best of the Company's knowledge, the
Company is not infringing upon or otherwise acting adversely to the right or
claimed right of any other person under or with respect to the foregoing and has
not received any communications alleging that the Company has violated or, by
conducting its business as proposed, would violate any of the patents,
trademarks, service marks, trade names, copyrights or other proprietary rights
of any other person or entity.

                                       6
<PAGE>

           (b)  The Company is at least a 50 percent owner of the entire right,
title and interest in and to the subject U.S. and foreign (excluding Israel)
patents and patent applications included in Schedule 3.13 (collectively, the
                                            -------------
"Properties"), and, in particular, is a 100 percent owner of the entire right,
-----------
title and interest in and to U.S. Application No. 08/893,403 and corresponding
PCT Application No. PCT/US971/12045.

           (c)  To the best knowledge of the Company, the Properties encompass
all patents and patent applications relating to communication systems that are
wholly or partially owned by Inline Connection Corporation.

           (d)  The Company has undertaken a reasonable investigation and has
determined that there are no claims, actions or proceedings, pending or to the
best knowledge of the Company threatened, or other information that challenges
the validity and/or the enforceability of any of the claims in the patents or of
any of the claims of the patent applications that may issue with respect to the
Properties.


           (e)  The Company will use commercially reasonable efforts to
prosecute (to the extent that the Company has a legal right to do so) all of the
pending patent applications listed in Schedule 3.13, with a view to obtaining
broad patent protection covering technology (known as "Overvoice") relating to
                                                       ---------
the simultaneous transmission of voice and data over a single traditional copper
telephone line at speeds of up to 300 times those of conventional 28.8k dial-up
modems and to enable a user to have both designated high-speed Internet access
and complete use of his telephone simultaneously over one traditional telephone
line.

           (f)  There are no encumbrances, third party or otherwise, against any
of the Properties.


     3.14  Tax Matters. The Company, any predecessor of the Company and all
           -----------
current and former members for income tax purposes of any affiliated group of
corporations of which the Company or any such predecessor is or has been a
member, Cleartel, Inc. and Cleartel, L.P. (collectively, the "Taxpayers") have
                                                              ---------
duly filed all tax reports and returns required to be filed by them or have
requested and obtained appropriate extensions, including all federal, state,
local and foreign tax returns and reports.  The Taxpayers have paid in full all
taxes required to be paid by such Taxpayers before such payment became
delinquent or have otherwise paid any required interest and penalties relating
thereto or have made adequate provision, in conformity with U.S. GAAP
consistently applied, for the payment of such taxes as well as taxes which may
subsequently become due.  There are no audits known by the Company to be pending
of the tax returns of the Company or any other Taxpayer, and there are no claims
known by the Company or any Taxpayer which have been or maybe asserted relating
to any tax returns filed for any year which if determined adversely would result
in the assertion by any governmental agency of any material deficiency.

     3.15  Investment Company.  Immediately following the Closing, after giving
           ------------------
effect to the transactions contemplated hereby, neither the Company nor any
person, firm or entity

                                       7
<PAGE>

controlling, controlled by or under common control with the Company will be an
"investment company" within the meaning of the Investment Company Act of 1940,
as amended.

     3.16  Employer Matters.  Except as disclosed in the Company SEC Reports, or
           ----------------
as set forth in Schedule 3.16, the Company does not have any employment
                -------------
contracts with any of its employees that are not terminable at will and does not
have any collective bargaining agreement covering any of its employees. The
Company is not aware of any labor organization activity involving its employees.
The Company is in compliance in all material respects with the requirements of
(i) the Employee Retirement Income Security Act of 1974, as amended, and (ii)
the Internal Revenue Code of 1986, as amended, in each case relating to employee
benefit plans, programs and agreements. Neither the execution nor delivery of
this Agreement, nor the carrying on of the Company's business by the employees
of the Company will, to the best of the Company's knowledge, conflict with or
result in a breach of the terms, conditions or provisions of, or constitute a
default under, any contract, covenant or instrument under which any of such
employees is now obligated, except for breaches or conflicts which would not
have a material adverse effect on the Company's business, operations or
financial condition.

     3.17  Condition of System.  All material properties, equipment and systems
           -------------------
of the Company are in good repair, working order and condition and are in
material compliance with all standards and rules imposed (i) by any governmental
agency or authority in which such properties, equipment and/or systems are
located or operated and (ii) under any agreements with customers.

     3.18  Fees; License Compliance.  The Company has paid all material
           ------------------------
franchise, license or other fees and charges which have become due in respect of
its business and has made appropriate provisions as is required by GAAP for any
such fees and charges which have accrued.  The Company has duly secured all
necessary and material permits, licenses, consents and authorizations from and
have filed all required and material registrations, applications, reports and
other documents with, the appropriate governmental agencies, authorities and
commissions and other entities exercising jurisdiction over the business of the
Company.  All of the Company's licenses, including licenses held through or by
affiliated entities, are valid and in full force and effect without conditions
except such conditions as are generally applicable to holders of licenses.  To
the best of the Company's knowledge, no event has occurred and is continuing
which could result in the termination, revocation or adverse modification of any
license.  The Company does not have any reason to believe that its licenses,
including licenses held through or by its affiliated entities, will not be
renewed in the ordinary course.

      3.19  No Conflict of Interest. Except as disclosed in the Company SEC
            -----------------------
Reports or as set forth in Schedule 3.19, the Company is not indebted, directly
                           -------------
or indirectly, to any of its officers, directors or stockholders or to their
respective spouses or children, in any amount whatsoever, other than for normal
travel advances or reimbursement for normal business expenses; and none of such
officers, directors or stockholders, or any members of their immediate families
is indebted to the Company.  Schedule 3.19 sets forth a description of all
                             -------------
transactions since January 1, 1997, between the Company and any of its officers,
directors and stockholders, and their respective spouses and children in which
such persons had a direct or indirect material interest which are not disclosed
in the Company SEC Reports.

                                       8
<PAGE>

     3.20  Agreements; Action.
           ------------------

           (a)  Except as disclosed in the Company SEC Reports or as set forth
in Schedule 3.20, there are no agreements, understandings, instruments,
contracts, judgments, orders, writs or decrees to which the Company is a party
or by which it is bound that involve (i) obligations (contingent or otherwise)
of, or payments to, the Company in excess of $50,000, other than transactions in
the ordinary cause of business, (ii) the license of any patent, copyright, trade
secret or other proprietary rights to or from the Company, (iii) provisions
restricting or affecting the development, manufacture or distribution of the
Company's products or services or (iv) indemnification by the Company with
respect to infringements of proprietary rights.

           (b)  The Company is not a party to and is not bound by any contract,
agreement or instrument, and is not subject to any restriction under its
Restated Certificate or its By-Laws, that materially adversely affects its
business as now conducted or as proposed to be conducted, or its properties or
financial condition.

           (c)  Except as disclosed in the Company SEC Reports or as provided
in Schedule 3.20, the Company has not granted or agreed to grant any
   -------------
registration rights, including piggy back rights, to any person or entity.

     3.21  Complete Copies of Requested Documents.  The Company has delivered or
           --------------------------------------
made available true and complete copies of each document that has been
reasonably requested by Purchaser.

     3.22  Full Disclosure.  Neither this Agreement (including all Exhibits
           ---------------
hereto) nor any of the other agreements or instruments contemplated to be
executed and delivered by the Company in connection with this Agreement contain
any untrue statement of material fact; and none of such documents omits to state
any material fact necessary to make any of the representations, warranties or
other statements or information contained therein not misleading.

                                   Section 4

                  Representations and Warranties of Purchaser
                  -------------------------------------------

     Purchaser hereby represents and warrants to the Company with respect to the
purchase of the Shares by Purchaser, as follows:

     4.1   Experience; Speculative Nature of Investment.  Purchaser has
           --------------------------------------------
substantial experience in evaluating and investing in private placement
transactions of securities in companies similar to the Company so that it is
capable of evaluating the merits and risks of its investment in the Company and
has the capacity to protect its own interests.  Purchaser acknowledges that its
investment in the Company is highly speculative and entails a substantial degree
of risk and Purchaser is in a position to lose the entire amount of such
investment.

     4.2   Investment.  Purchaser is acquiring the Shares and the underlying
           ----------
Conversion Stock for investment for its own account, not as a nominee or agent,
and not with the view to, or for resale in connection with, any distribution
thereof.  Purchaser is an "accredited investor" within the meaning of Regulation
D, Rule 501(a), promulgated by the SEC.

                                       9
<PAGE>

     4.3   Restricted Securities. Purchaser acknowledges that the Shares and the
           ---------------------
underlying Conversion Stock must be held indefinitely unless subsequently
registered under the Securities Act or unless an exemption from the registration
requirements of the Securities Act is available.

     4.4   Access to Data.  Purchaser has had an opportunity to discuss the
           --------------
Company's business, management and financial affairs with the Company's
management.  Purchaser has had an opportunity to ask questions of officers of
the Company, which questions were answered to its satisfaction.  Purchaser
understands that such discussions, as well as any other written information
issued by the Company, were intended to describe certain aspects of the
Company's business and operations, but were not an exhaustive description.

     4.5   Authority; Binding Nature of Agreements.
           ---------------------------------------

           (a)  The execution, delivery and performance of this Agreement and
all other agreements and instruments contemplated to be executed and delivered
by Purchaser in connection herewith have been duly authorized by all necessary
corporate action on the part of Purchaser and its board of directors. Purchaser
has all requisite legal and corporate power and authority to purchase the Shares
hereunder.

           (b)  This Agreement and all other agreements and instruments
contemplated to be executed and delivered by Purchaser in connection herewith
constitute the legal, valid and binding obligations of Purchaser, enforceable
against Purchaser in accordance with their terms, except to the extent that
enforceability may be limited by applicable bankruptcy, merger, insolvency,
moratorium, fraudulent conveyance or other laws affecting the enforcement of the
rights of creditors and other obligees generally and by general principles of
equity regardless of whether such enforceability is considered in a proceeding
in law or equity, and by limitations on indemnification due to public policy
considerations.

           (c)  There is no pending proceeding, and, to Purchaser's knowledge,
no person has threatened to commence any proceeding that challenges, or that may
have the effect of preventing, delaying, making illegal or otherwise interfering
with Purchaser's ability to comply with or perform its obligations and covenants
under this Agreement and under all other agreements and instruments contemplated
to be executed and delivered by Purchaser in connection herewith, and, to the
knowledge of Purchaser, no event has occurred, and no claim, dispute or other
condition or circumstance exists, that might directly or indirectly give rise to
or serve as a basis for the commencement of any such proceeding.

     4.6   Brokers or Finders.  Purchaser has not engaged any broker, finder or
           ------------------
agent, and the Company has not, and will not, incur, directly or indirectly, as
a result of any action taken by Purchaser, any liability for brokerage or
finders' fees or agents' commissions or any similar charges in connection with
the purchase of the Shares.  In the event that the preceding sentence is in any
way inaccurate, the Purchaser agrees to indemnify and hold harmless the Company
from any liability for any commission or compensation in the nature of any such
fee (and the costs and expenses of defending against such liability) for which
the Company or any of its officers, directors, employees or representatives, is
held responsible.

     4.7   Tax Liability. Purchaser has reviewed with its own tax advisors the
           -------------
federal, state, local and foreign tax consequences of this investment and the
transactions contemplated by this Agreement. With respect to such matters,
Purchaser has relied solely on such advisors and

                                       10
<PAGE>

not on any statements or representations of the Company or any of its
representatives other than the representations and warranties set forth herein.
Purchaser understands that it (and not the Company) shall be responsible for its
own tax liability that may arise as a result of this investment or the
transactions contemplated by this Agreement.

                                   Section 5

                 Conditions to Purchaser's Obligations to Close
                 ----------------------------------------------

     Purchaser's obligations to purchase the Shares at the Closing are, unless
waived by Purchaser, subject to the fulfillment of the following conditions:

     5.1   Representations and Warranties Correct.  The representations and
           --------------------------------------
warranties made by the Company in Section 3 hereof shall be true and correct in
all material respects as of the Closing Date.

     5.2   Covenants.  All covenants and agreements contained in this Agreement
           ---------
and the other agreements and instruments contemplated herewith to be performed
by the Company on or prior to the Closing shall have been performed or complied
with in all materials respects.

     5.3   Blue Sky.  The Company shall have obtained all necessary Blue Sky law
           --------
permits and qualifications, or have the availability of exemptions therefrom,
required by any state for the offer and sale of the Shares.

     5.4   Registration Rights Agreement.  The Company shall have executed and
           -----------------------------
delivered the Registration Rights Agreement as of the Closing Date.



     5.5   Opinion of Company's Counsel.  The Purchaser shall have received
           ----------------------------
from Morrison & Foerster, LLP, counsel to the Company, an opinion addressed to
it, dated as of the Closing Date, is substantially the form of Exhibit D.
                                                               ---------

     5.6   Commercial Agreement.  The Company shall have executed and delivered
           --------------------
to Purchaser, as of the Closing Date, a Commercial Agreement, in form and
substance mutually acceptable to the Company and Purchaser  (the "Commercial
Agreement") and the Purchaser shall have executed and delivered to the Company
the Commercial Agreement as of the Closing Date.

     5.7   HSR Compliance.  All waiting periods applicable to the purchase of
           --------------
the Shares under the HSR Act shall have been terminated or expired.

                                   Section 6

                  Conditions to Company's Obligations to Close
                  --------------------------------------------

     The Company's obligation to sell and issue the Shares at the Closing is,
unless waived by the Company, subject to the fulfillment of the following
conditions:

     6.1   Representations and Warranties Correct.  The representations and
           --------------------------------------
warranties made by Purchaser in Section 4 hereof shall be true and correct in
all material respects as of the Closing Date.

                                       11
<PAGE>

     6.2   Covenants.  All covenants and agreements contained in this Agreement
           ---------
and all other agreements and instruments contemplated herewith to be performed
by Purchaser on or prior to the Closing shall have been performed or complied
with in all material respects.

     6.3   Blue Sky.  The Company shall have obtained all necessary blue sky
           --------
law permits and qualifications, or have the availability of exemptions
therefrom, required by any state for the offer and sale of the Shares.

     6.4   Registration Rights Agreement.  Purchaser shall have executed and
           -----------------------------
delivered the Registration Rights Agreement as of the Closing Date.


     6.5   Commercial Agreement.  Purchaser shall have executed and delivered
           --------------------
to the Company the Commercial Agreement as of the Closing Date.


     6.6   HSR Compliance.  All waiting periods applicable to the purchase of
           --------------
the Shares under the HSR Act shall have been terminated or expired.


                                   Section 7

                            Confidential Information
                            ------------------------

     7.1   Confidential Company Information. Purchaser covenants and agrees that
           --------------------------------
it shall maintain the confidentiality of all nonpublic information related to
the Company made available to it and/or any of its representatives by or on
behalf of the Company ("Confidential Company Information"). Purchaser further
                        --------------------------------
covenants and agrees that it shall not disclose any Confidential Company
Information to any person or entity, other than its officers, directors,
employees, attorneys, accountants and other agents with a legitimate need for
such information (which individuals and entities Purchaser shall cause to comply
with this Section 7.1), except as required by law, without the prior written
consent of the Company.  Purchaser agrees that violation of this Section 7.1
would cause immediate and irreparable damage to the business of the Company, and
consents to the entry of immediate and permanent injunctive relief for any
violation hereof.

     7.2   Confidential Purchaser Information.  The Company covenants and agrees
           ----------------------------------
that it shall maintain the confidentiality of all nonpublic information related
to Purchaser made available to it and/or any of its representatives by or on
behalf of Purchaser ("Confidential Purchaser Information").  The Company further
                      ----------------------------------
covenants and agrees that it shall not disclose any Confidential Purchaser
Information to any person or entity, other than its officers, directors,
employees, attorneys, accountants and other agents with a legitimate need for
such information (which individuals and entities the Company shall cause to
comply with this Section 7.2), except as required by law, without the prior
written consent of Purchaser.  The Company agrees that violation of this Section
7.2 would cause immediate and irreparable damage to the business of Purchaser,
and consents to the entry of immediate and permanent injunctive relief for any
violation hereof.


                                       12
<PAGE>

                                   Section 8

                                Other Covenants
                                ---------------

     8.1   Future Issuances.  Notwithstanding any other provision to the
           -----------------
contrary herein or in the Certificate of Designation, in addition to the voting
rights set forth in the Certificate of Designation, the written consent of the
Purchaser shall be required to approve any issuance and sale by the Company of
equity securities in the Company, to any person or entity which the Purchaser
and the Company  mutually determine to be a competitor of the Purchaser;
provided, however, that no separate vote as contemplated by this Section 8.1
shall be required if any such issuance or sale is subject to Section 6(b)(iii)
of the Certificate of Designation; and provided further that this Section 8.1
shall immediately become null and void at the earlier of such time that: (i) the
Commercial Agreement is terminated or (ii) Purchaser owns of record no Shares..

                                   Section 9

                                 Miscellaneous
                                 -------------

     9.1   Governing Law.  This Agreement shall be governed in all respects by
           -------------
the internal laws of the State of Delaware, without regard to the conflicts of
laws provisions thereof.

     9.2   Jurisdiction; Jury Trial Waiver.  The parties hereto hereby
           -------------------------------
irrevocably submit to the exclusive jurisdiction of the state and federal courts
located in the State of Delaware with respect to any action or proceeding
arising out of this Agreement or in any way arising here from or relating
hereto.  THE PARTIES HEREBY WAIVE ALL RIGHTS TO TRIAL BY JURY IN ANY ACTION OR
PROCEEDING INSTITUTED BY ANY PARTY AGAINST ANY OTHER PARTY ARISING ON, OUT OF OR
BY REASON OF THIS AGREEMENT, ANY ALLEGED TORTIOUS CONDUCT BY ANY PARTY OR IN ANY
WAY, DIRECTLY OR INDIRECTLY, ARISING OUT OF OR RELATED TO THE RELATIONSHIP
BETWEEN THE PARTIES.

     9.3   Survival.  The representations, warranties, covenants and agreements
           --------
made herein shall survive any investigation made by Purchaser and the closing of
the transactions contemplated hereby.

     9.4   Successors and Assigns.  Except as otherwise provided herein, the
           ----------------------
provisions hereof shall inure to the benefit of, and be binding upon, the
successors, permitted assigns, heirs, executors and administrators of the
parties hereto; provided, however, that the rights of Purchaser to purchase the
                --------  -------
Shares shall not be assignable without the prior written consent of the Company.

     9.5   Entire Agreement; Amendment.  This Agreement and the other documents
           ---------------------------
delivered pursuant hereto at the Closing constitute the full and entire
understanding and agreement between the parties with regard to the subject
matter hereof and thereof, and no party shall be liable or bound to any other
party in any manner by any warranties, representations or covenants except as
specifically set forth herein or therein.  Except as expressly provided herein,
neither this Agreement nor any term hereof may be amended, waived, discharged or
terminated other than by a written instrument signed by the party against whom
enforcement of any such amendment, waiver, discharge or termination is sought;
provided, however, that Purchaser may, with the Company's prior written consent,
--------  -------
waive, modify or amend any provision hereof governing the rights and obligations
of Purchaser.

                                       13
<PAGE>

     9.6   Notice, etc.  All notices and other communications required or
           ------------
permitted hereunder shall be in writing and shall be mailed by registered or
certified mail, postage prepaid, or otherwise delivered by hand or by messenger,
addressed (a) if to Purchaser, at Purchaser's address:  5400 Bayfront Plaza,
Santa Clara, California  95052, Attention: Chief Financial Officer, with a copy
to Wilson Sonsini Goodrich & Rosati, P.C., 650 Page Mill Road, Palo Alto,
California 94304-1050, Attention:  Aaron J. Alter, or at such other address as
Purchaser shall have furnished to the Company in writing, or (b) if to the
Company, at the Company's address: 1255 22nd Street, N.W., Washington, D.C.
20037, or at such other address as the Company shall have furnished to
Purchaser, and addressed to the attention of the Chief Executive Officer, with a
copy to Morrison & Foerster LLP, 2000 Pennsylvania Avenue, N.W., Washington,
D.C.  20006, Attention:  Morris F. DeFeo, Jr..  Each such notice or other
communication shall for all purposes of this Agreement be treated as effective
or having been given when delivered if delivered personally, or, if sent by
mail, at the earlier of its receipt or seventy-two (72) hours after the same has
been deposited in a regularly maintained receptacle for the deposit of the
United States mail, addressed and mailed as aforesaid.

     9.7   Delays or Omissions. Except as expressly provided herein, no delay or
           -------------------
omission to exercise any right, power or remedy accruing to either party to this
Agreement upon any breach or default of the other party under this Agreement,
shall impair any such right, power or remedy of such nondefaulting party nor
shall it be construed to be a waiver of any such breach or default, or an
acquiescence therein, or of or in any similar breach or default thereafter
occurring; nor shall any waiver of any single breach or default be deemed a
waiver of any other breach or default theretofore or thereafter occurring. Any
waiver, permit, consent or approval of any kind or character on the part of
either party of any breach or default under this Agreement, or any waiver on the
part of either party of any provisions or conditions of this Agreement, must be
in writing and shall be effective only to the extent specifically set forth in
such writing. All remedies, either under this Agreement or by law or otherwise
afforded to either party to this Agreement, shall be cumulative and not
alternative.

     9.8   Counterparts.  This Agreement may be executed in any number of
           ------------
counterparts, each of which shall be enforceable against the party actually
executing such counterparts, and all of which together shall constitute one
instrument.

     9.9   Severability.  In the event that any provision of this Agreement
           ------------
becomes or is declared by a court of competent jurisdiction to be illegal,
unenforceable or void, this Agreement shall continue in full force and effect
without said provision; provided, however, that no such severability shall be
effective if it materially changes the economic benefit of this Agreement to
either party.

     9.10  Titles and Subtitles.  The titles and subtitles used in this
           --------------------
Agreement are used for convenience only and are not considered in construing or
interpreting this Agreement.

     9.11  Expenses.  The Company and Purchaser shall bear their own expenses
           --------
with respect to this Agreement and the transactions contemplated hereby.



                   [Signatures appear on the following page]

                                       14
<PAGE>

     The foregoing Series G Preferred Stock Purchase Agreement is hereby
executed as of the date first above written.

                                         "COMPANY"

                                         CAIS INTERNET, INC.


                                         By:  /s/ Ulysses G. Auger., II CEO
                                             ------------------------------

                                         "PURCHASER"

                                         3COM CORPORATION


                                         By:  /s/ Janice M. Roberts
                                              ________________________________
<PAGE>

                                   EXHIBIT A
                                   ---------


               Amended and Restated Certificate of Incorporation
<PAGE>

                                   EXHIBIT B
                                   ---------

                                    By-Laws
<PAGE>

                                   EXHIBIT C
                                   ---------

                          Certificate of Designation
<PAGE>

                                   EXHIBIT D
                                   ---------

                         Opinion of Company's Counsel
<PAGE>

                                   SCHEDULES
                                      TO
                  SERIES G PREFERRED STOCK PURCHASE AGREEMENT


          The following schedules refer to the Series G Preferred Stock Purchase
Agreement (the "Agreement"), dated as of March 20, 2000, between CAIS Internet,
Inc. (the "Company") and 3Com Corporation (the "Purchaser").

          Nothing in the following schedules is intended to broaden the scope of
any representation or warranty contained in the Agreement or to create any
covenant on the part of the Company.

          To the extent more than one representation and warranty contained in
the Agreement requires the same disclosure, the appearance of such disclosure on
any single item herein shall serve as disclosure for all other representations
and warranties to which such disclosure applies.  The failure by the Company to
cross-reference any disclosure on any particular schedule shall not constitute a
breach by the Company of the applicable representation or warranty as long as
the matter is disclosed elsewhere in these schedules.

          Inclusion of any item in the schedules (1) does not represent a
determination by the Company that such item is material nor shall it be deemed
to establish a standard of materiality (it being the intent that the Company
shall not be penalized for having disclosed more than it may be required by the
terms of the Agreement), (2) does not represent a determination by the Company
that such item did not arise in the ordinary course of business, and (3) shall
not constitute, or be deemed to be, an admission concerning such item by the
Company.  The items in the schedules are descriptions or brief summaries of
certain aspects of the Company and the Company's business and are necessarily
not complete.  Accordingly, the schedules are qualified in their entirety by
reference to the more detailed information in documents attached hereto or
previously delivered or made available to the Purchaser and its representatives.

          Capitalized terms used but not defined herein shall have the same
meanings ascribed to them in the Agreement.  The headings in the following
schedules are for reference only and shall not affect the disclosures contained
therein.
<PAGE>

                                 SCHEDULE 3.13
                                      TO
                  SERIES G PREFERRED STOCK PURCHASE AGREEMENT


     As of the date hereof, the Company owns at least a fifty percent (50%) of
the entire right, title and interest in and to the following subject U.S. and
foreign (excluding Israel) patents and patent applications:


     The Company is a licensee and joint-owner of patents and patent
applications of Inline Connection Corporation ("Inline") relating to the
OverVoice technology.  The Company, together with Inline, has five U.S. patents
and eleven U.S. patent applications.  Two of these eleven patent applications
has recently been allowed and, therefore, is expected to become patents in the
next several months.

     Together with Inline, which also owns 50%, the Company owns 50% of all U.S.
and foreign (with the exception of Israel) patents and patent applications
relating to the OverVoice technology, except for certain patent applications
relating to the OverVoice technology filed in the European Patent Office,
Australia, Brazil, Canada, China, Czech Republic, Eurasia, Hungary, Indonesia,
Japan, North Korea, South Korea, Mexico, Norway, New Zealand, Poland and
Singapore, as to which the Company owns 100%.

     The first U.S. patent granted relates to transmission of video over active
voice telephone wires.  Related patents have also been obtained in Canada and
from the European Patent Office, covering Germany, France and the United
Kingdom.  In addition, a patent was issued in South Korea and a division
application was filed in Europe.

     The second U.S. patent granted relates to some or all aspects of the
following systems, among others:

 .      provision of high-speed Internet service through the communication of
       Ethernet signals over the active telephone wiring in residences, hotels,
       apartment buildings and similar structures;

 .      provision of video services over the telephone writing in the same
       structures;

 .      provision of webTV-type services over the telephone wiring in these
       structures;

 .      creation of a standard Ethernet network, using existing telephone wiring,
       among all personal computers in a structure; and

 .      communication of Ethernet signals over 1,000 feet over a single active
       telephone line.

     Novel ideas are embodied in many of the different parts that make up these
systems.  Among these parts are:
<PAGE>

 .       different electronic processes for converting the video and data
        signals; special connectors that are easy to install, convenient to use
        and promote smooth signal flow across the wiring;

 .       different arrangements of the components to facilitate the operation of
        the systems; and

 .       special "command and control" procedures that help implement the
        different applications.


     Pursuant to the Company's license agreement with Inline, the Company has
the exclusive right to make, use and sell the OverVoice technology for all
structures in the United States, except for single family residential units and
certain food establishments, for which the Company has non-exclusive rights.
The Company further has the exclusive right to make, use and sell under all
foreign patents and patent applications relating to the OverVoice technology for
all structures except for single family residential units and certain food
establishments, for which the Company has non-exclusive rights, with the
exception of Israel, which Inline reserved for itself.

     Business Anywhere USA, Inc. ("BAC"), a subsidiary of the Company, is the
assignee of US Patent 5,901,067 and US Patent Application 09/088,213.  This
intellectual property was assigned to BAC by Kim Kao.

     Please see the attached documents for a list of patents and patent
applications held by CAIS Software Solutions, Inc., a subsidiary of the Company.
<PAGE>

                                 SCHEDULE 3.16
                                      TO
                  SERIES G PREFERRED STOCK PURCHASE AGREEMENT


     Except as set forth in the Company SEC Reports or as set forth below, as of
the date hereof, the Company does not have any (i) employment contracts with any
of its employees that are not terminable at will, or (ii) collective bargaining
agreement covering any of its employees.

I.   Employment Contracts
     --------------------

     None.

II.  Collective Bargaining Agreements
     --------------------------------

     None.

                                       23
<PAGE>

                                 SCHEDULE 3.19
                                      TO
                  SERIES G PREFERRED STOCK PURCHASE AGREEMENT


     Except as set forth in the Company SEC Reports or as set forth below, as of
the date hereof, the Company is not indebted, directly or indirectly, to any of
its officers, directors or stockholders or to their respective spouses or
children, in any amount whatsoever, other than for normal travel advances or
reimbursement for normal business expenses; and none of such officers, directors
or stockholders, or any member of their immediate families is indebted to the
Company.


     None.


     Additionally, set forth below, is a description of all transactions since
January 1, 1996, between the Company and any of its officers, directors and
stockholders and their respective spouses and children in which such persons had
a direct or indirect material interest which are not disclosed in the Company
SEC Reports.


     None.

                                       24