<PAGE>
                                                                     Exhibit 4.9


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                           USN COMMUNICATIONS, INC.


                      14% SENIOR DISCOUNT NOTES DUE 2003


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                         SUPPLEMENTAL INDENTURE NO. 3

                           Dated as of March 5, 1998


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                        HARRIS TRUST AND SAVINGS BANK,

                                    Trustee


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<PAGE>

     SUPPLEMENTAL INDENTURE NO. 3, dated as of March 5, 1998 (the "Supplement"),
to the Indenture, dated as of September 30, 1996, as supplemented on March 17,
1997 and August 12, 1997 (as supplemented, the "Indenture"), between USN
COMMUNICATIONS, INC. (formerly, United USN, Inc.), a Delaware corporation (the
"Company"), and HARRIS TRUST AND SAVINGS BANK, as trustee hereunder (the
"Trustee"). Capitalized terms used herein but not otherwise defined shall have
the meanings ascribed to such terms in the Indenture.

                            RECITALS OF THE COMPANY

     WHEREAS, the Company and the Trustee have entered into the Indenture
pursuant to which the Company has issued $48,500,000 aggregate principal amount
at Stated Maturity of its 14% Senior Discount Notes due 2003 (the "Notes");

     WHEREAS, Section 9.01 of the Indenture provides that the Company and the
Trustee may, at any time, without notice to or consent of any Holders of the
outstanding Notes, enter into one or more indentures supplemental thereto for
the purpose of curing any ambiguity therein, or correcting or supplementing any
provision thereof or adding any other provisions with respect to matters or
questions arising under the Indenture, provided that such action will not
adversely affect the interests of the Holders of the outstanding Notes in any
material respect;

     WHEREAS, all acts and things prescribed by the Indenture and by law
necessary to make this Supplement a valid instrument legally binding on the
Company and the Trustee, in accordance with its terms, have been duly done and
performed; and

     WHEREAS, all conditions precedent to amend or supplement the Indenture have
been met.

     NOW, THEREFORE, each party agrees, for the benefit of the other parties and
for the equal and ratable benefit of the Holders of the Notes, to the deletions,
amendments and modifications set forth below which will become operative
pursuant to the terms hereof.



                                       2
<PAGE>

                                   ARTICLE I

                                  AMENDMENTS

          Section 1.01 Deletions, Amendments and Modifications to Article I.

          (1)   The following term is inserted in Section 1.01 of the Indenture
as a new definition.

               ""Second Consent Agreement" means the Consent Agreement, dated as
     of January 7, 1998, by and among the Company, Merrill Lynch Global
     Allocation Fund, Inc. and Merrill Lynch Equity/Convertible Series (Global
     Allocation Portfolio)."

          2. The definition of "New Senior Notes" is hereby replaced in its
entirety with the following:

               ""New Senior Notes" means up to $152,725,000 aggregate principal
     amount at stated maturity of senior notes to be issued pursuant to the New
     Senior Note Indenture and up to $52,000,000 aggregate principal amount at
     stated maturity of additional New Senior Notes that may be issued pursuant
     to the New Senior Note Indenture upon the exchange of Senior Notes, in
     whole or in part, by the holders thereof, for New Senior Notes pursuant to
     the Consent Agreement, as amended by the Second Consent Agreement."


                                   ARTICLE II

                                 MISCELLANEOUS

          Section 2.01 Effect of this Supplement. This Supplement is
supplemental to the Indenture and does and shall be deemed to form a part of,
and shall be construed in connection with and as part of, the Indenture for any
and all purposes. Except as specifically modified herein, the Indenture and the
Notes are in all respects ratified and confirmed and shall remain in full force
and effect in accordance with their terms.

          Section 2.02 Trustee. Except as otherwise expressly provided herein,
no duties, responsibilities or liabilities are assumed, or shall be construed to
be



                                       3
<PAGE>

assumed, by the Trustee by reason of this Supplement. This Supplement is
executed and accepted by the Trustee subject to all the terms and conditions set
forth in the Indenture with the same force and effect as if those terms and
conditions were repeated at length herein and made applicable to the Trustee
with respect hereto. The Trustee assumes no responsibility for the recitals
contained herein, which shall be taken as statements of the Company, and makes
no representation as to the validity or sufficiency of this Supplement.

          Section 2.03 GOVERNING LAW. THIS SUPPLEMENT AND THE NOTES SHALL BE
GOVERNED BY, AND CONSTRUED IN ACCORDANCE WITH, THE LAWS OF THE STATE OF NEW
YORK, EXCEPT WITH REGARD TO PRINCIPLES OF CONFLICTS OF LAWS, APPLICABLE TO
AGREEMENTS MADE AND TO BE PERFORMED IN SAID STATE.

          Section 2.04 Counterparts. This Supplement may be executed in any
number of counterparts and by the parties thereto in separate counterparts, each
of which when so executed shall be deemed to be an original and all of which
taken together shall constitute one and the same agreement.

          Section 2.05 Severability. In case any provision in this Supplement,
the Indenture or in the Notes shall be invalid, illegal or unenforceable, the
validity, legality and enforceability of the remaining provisions shall not in
any way be affected or impaired thereby.

          Section 2.06 Titles and Headings. The titles and headings in this
Supplement are solely for convenience of reference and will not be given any
effect in the construction or interpretation of this Supplement.



                                       4
<PAGE>

          IN WITNESS WHEREOF, the parties hereto have caused this Supplement to
be duly executed, and their respective corporate seals to be hereunto affixed
and attested, all as of the day and year first above written.


                                       USN COMMUNICATIONS, INC.



                                       By
                                         --------------------------------------
                                       Name:
                                       Title:



[Corporate Seal]


Attest


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                                       HARRIS TRUST AND SAVINGS BANK,
                                       as Trustee




                                       By
                                         --------------------------------------
                                       Name:
                                       Title:



[Corporate Seal]


Attest


---------------------
<PAGE>

STATE OF ILLINOIS       )
                        )  SS.:
COUNTY OF COOK          )


          On the ___ day of March, 1998, before me personally came
_______________, to me known, who being by me duly sworn, did depose and say
that he is _______________________ of USN Communications, Inc., one of the
corporations described in and which executed the foregoing instrument; that he
knows the seal of said corporation; that the seal affixed to said instrument is
such corporate seal; that it was so affixed by authority of the Board of
Directors of said corporation, and that he signed his name thereto by like
authority.




                                       ----------------------------------------
                                       Notary Public


                                       State of Illinois
                                       My commission expires


[Seal]

<PAGE>

STATE OF ILLINOIS       )
                        )  SS.:
COUNTY OF COOK          )


          On the ___ day of March, 1998, before me personally came
_______________, to me known, who being by me duly sworn, did depose and say
that he is _______________________ of Harris Trust and Savings Bank, the Trustee
described in and which executed the foregoing instrument; that he knows the seal
of said corporation; that the seal affixed to said instrument is such corporate
seal; that it was so affixed by authority of the Board of Directors of said
corporation, and that he signed his name thereto by like authority.




                                       ----------------------------------------
                                       Notary Public


                                       State of Illinois
                                       My commission expires



[Seal]