<PAGE>   1
                                                                EXHIBIT 10.16.3



                            HERITAGE OPERATING, L.P.

                           THIRD AMENDMENT AGREEMENT


         Re:   Note Purchase Agreement dated as of June 25, 1996
             Note Purchase Agreement dated as of November 19, 1997

                                                                     Dated as of
                                                                    May 31, 2000

To each of the Holders named
  in Schedule 1 to this Consent and
  Third Amendment Agreement

Ladies and Gentlemen:

         Reference is made to

         (i) the Note Purchase Agreement dated as of June 25, 1996 (the
"Original 1996 Agreement"), among Heritage Operating, L.P., a Delaware limited
partnership (the "Company") and the Purchasers named in the Purchaser Schedule
attached thereto, as amended by a First Amendment Agreement (the "First
Amendment Agreement") dated as of October 15, 1998 and a Second Amendment
Agreement (the "Second Amendment Agreement") dated as of September 1, 1999
(said Original 1996 Agreement, as amended by the First Amendment Agreement and
the Second Amendment Agreement, being hereinafter referred to as the
"Outstanding 1996 Agreement") under and pursuant to which the Company issued,
and there are presently outstanding, $120,000,000 aggregate principal amount of
its 8.55% Senior Secured Notes due 2011 (the "1996 Notes"); and

        (ii) the Note Purchase Agreement dated as of November 19, 1997 (the
"Basic 1997 Agreement"), among the Company and the Purchasers named in the
Initial Purchaser Schedule attached thereto, as amended by the First Amendment
Agreement and the Second Amendment Agreement (said Basic 1997 Agreement, as so
amended, being hereinafter referred to as the "Amended Basic 1997 Agreement"),
under and pursuant to which the Company issued, and there are presently
outstanding, $12,000,000 aggregate principal amount of its 7.17% Series A
Senior Secured Notes due November 19, 2009 (the "Series A Notes") and
$20,000,000 aggregate principal amount of its 7.26% Series B Senior Secured
Notes due November 19, 2012 (the "Series B Notes"), as supplemented by the
First Supplemental Note Purchase Agreement dated as of March 13, 1998 the
"First Supplemental Agreement" among the Company and the Purchasers named in
the Supplemental Purchaser Schedule attached thereto, under and pursuant to
which (a) the Company issued $5,000,000 aggregate principal amount of its 6.50%
Series C




<PAGE>   2

Senior Secured Notes due March 13, 2007 (the "Series C Notes"), $4,285,714.29
of which are presently outstanding, and (b) the Company issued, and there are
presently outstanding, (x) $5,000,000 aggregate principal amount of its 6.59%
Series D Senior Secured Notes due March 13, 2010 (the "Series D Notes") and (y)
$5,000,000 aggregate principal amount of its 6.67% Series E Senior Secured
Notes due March 13, 2013 (the "Series E Notes").

The Amended Basic 1997 Agreement, as supplemented by the First Supplemental
Agreement is hereinafter sometimes referred to as the "Outstanding 1997
Agreement". The Outstanding 1996 Agreement and the Outstanding 1997 Agreement
are hereinafter sometimes collectively referred to as the "Outstanding
Agreements". The 1996 Notes, Series A Notes, Series B Notes, Series C Notes,
Series D Notes and Series E Notes are hereinafter sometimes collectively
referred to as the "Outstanding Notes." Capitalized terms used herein without
definition shall have the respective meanings assigned to such terms in the
Outstanding Agreements.

         Exhibit A hereto contains letters of the Company dated May 11, 2000
and May 19, 2000 (collectively, the "Transaction Description"), describing
certain acquisitions by the Company, a proposed change in the ownership of the
General Partner, a proposed change in ownership of Common Units of the Master
Partnership and other matters set forth therein (collectively, the "Proposed
Reorganization").

         The Company now seeks your agreement to an amendment with respect to
the each of the Outstanding Agreements necessary in order to effect the
Proposed Reorganization. You are the owner and holder of the Outstanding Notes
set forth opposite your name on Schedule 1 hereto. The Company hereby requests
that from and after the satisfaction of each of the Conditions to Effectiveness
set forth in Article II below, said amendment shall be deemed to have been
given and said Outstanding Agreements shall be amended in the respects, but
only in the respects, hereinafter set forth.

                                   ARTICLE I
                      AMENDMENTS TO OUTSTANDING AGREEMENTS

        I-A. Section 6A of each of the Outstanding Agreements shall be amended
by (x) changing the period at the end of clause (ii) to a semicolon, (y) adding
the word "or" after such semicolon, and (z) adding the following new clause
(iii):

                  "(iii) Ratio of Adjusted Consolidated Funded Indebtedness to
         Adjusted Consolidated EBITDA. The ratio as at the end of any fiscal
         quarter of Adjusted Consolidated Funded Indebtedness to Adjusted
         Consolidated EBITDA to exceed 6.25 to 1.00."


         I-B. Section 7A(xv) of each of the Outstanding Agreements shall be
amended by (i) adding before clause (a) the phrase "any of the events described
in clauses (a), (b), (c) or (d)


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<PAGE>   3

shall occur:", (ii) adding before clause (b) the word "or" and (iii) deleting
clause (c) and inserting in lieu thereof the following new clauses (c) and (d):

          "(c) the Specified Entities shall own, directly or indirectly through
          Wholly-Owned Subsidiaries, in the aggregate less than 51% of the
          Capital Stock of the General Partner, or (d) either Designated
          Current Manager shall, at any time during the Lock-up Period
          applicable to such Designated Current Manager, own, directly or
          indirectly, less than 50% of the Common Units of the Master
          Partnership owned, directly or indirectly, by such Designated Current
          Manager immediately after giving effect to the Proposed
          Reorganization; or"

         I-C. Section 10B of each of the Outstanding Agreements shall be
amended by adding thereto, at the appropriate alphabetical position, the
following additional defined terms

                   "`Adjusted Consolidated EBITDA' shall mean, as of any date
         of determination for any applicable period, Consolidated EBITDA
         calculated

                            (x) with respect to the consolidated group
                  comprised of the General Partner, the Master Partnership and
                  the Company and its Subsidiaries (rather than with respect to
                  the consolidated group comprised of the Company and its
                  Subsidiaries), and

                            (y) as if the terms `Consolidated Non-Cash
                  Charges', `Consolidated Net Income', `Consolidated Interest
                  Expense', `Consolidated Income Tax Expense', `Asset Sale',
                  and `Asset Acquisition', were calculated with respect to the
                  consolidated group comprised of the General Partner, the
                  Master Partnership and the Company and its Subsidiaries
                  (rather than with respect to the consolidated group comprised
                  of the Company and its Subsidiaries)."

                  "`Adjusted Consolidated Funded Indebtedness' shall mean
         Consolidated Funded Indebtedness calculated with respect to the
         consolidated group comprised of the General Partner, the Master
         Partnership and the Company and its Subsidiaries (rather than with
         respect to the consolidated group comprised of the Company and its
         Subsidiaries).

                  "`Designated Current Managers' shall mean R. C. Mills and H.
         Michael Krimbill, current executive officers of the General Partner,
         together with, in the case of either such executive officer, the heirs
         of, and trusts for the benefit of family members controlled by, such
         executive officer."

                  "`Lock-Up Period' shall mean, with respect to any Designated
         Current Manager, the period from the date of the closing of the
         Proposed Reorganization to the earlier to occur of (x) the third
         anniversary of such closing, and (y) the first



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<PAGE>   4

         date on which such Designated Current Manager shall cease to be
         employed by the General Partner, the Master Partnership or any of
         their respective Affiliates."

                  "`Proposed Reorganization' shall have the meaning set forth
         in the introductory portion of the Third Amendment Agreement, dated as
         of May 31, 2000, with respect to this Agreement."

                  "`Specified Entities' shall mean any one or more of the
         following entities: (i) Atmos Energy Corporation, a Texas and Virginia
         corporation, (ii) Piedmont Natural Gas Company, Inc., a North Carolina
         corporation, (iii) AGL Resources, Inc., a Georgia corporation, and
         (iv) TECO Energy, Inc., a Florida corporation, or a Successor to any
         entity referred to in clause (i), (ii), (iii) or (iv) of this
         definition."

                  "`Successor' shall mean, with respect to a Specified Entity,
         any entity in which the holders of the Capital Stock of such Specified
         Entity outstanding immediately prior to a consolidation, acquisition
         or merger involving such Specified Entity hold, directly or indirectly
         through Wholly-Owned Subsidiaries, at least a majority of the Capital
         Stock immediately after such consolidation, acquisition or merger."

                                   ARTICLE II
                          CONDITIONS OF EFFECTIVENESS

         The effectiveness of this Third Amendment Agreement is subject to the
satisfaction of the following conditions:

                   (a) the Required Holders under each of the of Outstanding
         Agreements shall have consented to this Third Amendment Agreement as
         evidenced by their execution thereof;

                   (b) the requisite percentage of lenders under the Credit
         Agreement (the "Lenders") shall have agreed to all amendments
         necessary to effect the Proposed Reorganization and a copy thereof
         shall have been provided to the holders of the Outstanding Notes. In
         the event the Company agrees that the Lenders or holders of any of the
         Outstanding Notes shall be granted any additional or more restrictive
         financial or negative covenants or events of default than are imposed
         on the Company under the Outstanding Agreements, as amended hereby,
         the Company agrees that the holders of all other Outstanding Notes
         shall also be granted such more restrictive covenants or events of
         defaults;

                   (c) each of the holders of the Outstanding Notes shall have
         received an amendment fee from the Company in an amount equal to .10%
         of the aggregate principal



                                      -4-
<PAGE>   5

         amount of the Outstanding Notes held by such holder (the "Amendment
         Fee") and a Responsible Officer of the Company shall have certified to
         each such holder (the truth and accuracy of which certification shall
         constitute a Condition of Effectiveness) that the Lenders have
         received no amendment fees or other consideration (including increase
         in coupon) greater than the Amendment Fee;

                   (d) the Holders of the Outstanding Notes shall have received
         evidence that (i) the Master Partnership shall have transferred to the
         Company an equity contribution in the amount of at least $45,000,000
         (the "Equity Contribution"), and (ii) the entire amount of such Equity
         Contribution shall have been applied to the payment of outstanding
         Indebtedness of the Company;

                   (e) all counsel to the holders of the Outstanding Notes
         shall have been paid fees and expenses incurred in connection with
         this Third Amendment Agreement;

                   (f) materials reasonably satisfactory to the holders of the
         Outstanding Notes shall have been delivered evidencing that the
         Proposed Reorganization has become effective; and

                   (g) each of the Designated Current Managers shall have
         entered into an employment agreement to act as an executive manager of
         the General Partner for a period of at least three years, all as
         contemplated in the Proposed Reorganization.

                                  ARTICLE III

                         REPRESENTATIONS AND WARRANTIES

         In order to induce the holders of the Notes to enter into this Third
Amendment, the Company represents and warrants that, (a) no Event of Default
has occurred and is continuing; (b) after giving effect to the Proposed
Reorganization, no Event of Default shall have occurred; and (c) the
information set forth in the Transaction Description does not contain any
untrue statement of a material fact or omit to state a material fact necessary
in order to make the statements contained therein, in light of the
circumstances under which they were made, not misleading.

                                   ARTICLE IV

                                 MISCELLANEOUS

       IV-A. If the foregoing is acceptable to you, kindly note your acceptance
in the space provided below and upon satisfaction of the Conditions to
Effectiveness set forth in Article II above, your consent to the Proposed
Reorganization shall be deemed to have been given and the Outstanding
Agreements shall be amended as set forth above.

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<PAGE>   6

       IV-B. This Third Amendment Agreement may be executed by the parties
hereto individually, or in any combination of the parties hereto in several
counterparts, all of which taken together shall constitute one and the same
Third Amendment Agreement.

       IV-C. Except as amended hereby, all of the representations, warranties,
provisions, covenants, terms and conditions of the Outstanding Agreements shall
remain unaltered and in full force and effect and the Outstanding Agreements,
as amended hereby, are in all respects agreed to, ratified and confirmed by the
Company. The Company acknowledges and agrees that the granting of amendments
herein shall not be construed as establishing a course of conduct on the part
of the holders of the Outstanding Notes upon which the Company may rely at any
time in the future.

       IV-D. Upon the effectiveness of this Third Amendment Agreement, each
reference in each Outstanding Agreement and in other documents describing or
referencing such Outstanding Agreement to "this Agreement," "hereunder,"
"hereof," "herein," or words of like import referring to such Outstanding
Agreement, shall mean and be a reference to such Outstanding Agreement, as
amended hereby.

                                     Very truly yours,

                                     HERITAGE OPERATING, L.P.


                                     By Heritage Holdings, Inc., General Partner


                                     By
                                     Its
                                        ----------------------------------------


                                      -6-
<PAGE>   7


         The foregoing Third Amendment Agreement and the amendments referred to
therein are hereby accepted and agreed to as of May 31, 2000, and the
undersigned hereby confirms that on May 31, 2000 it held the aggregate
principal amount of Outstanding Notes of the Company set forth on Schedule 1
hereto and that on the date of execution hereof it continues to hold such
Outstanding Notes.

JOHN HANCOCK LIFE INSURANCE
  COMPANY


By
   ----------------------------------
   Its

JOHN HANCOCK VARIABLE LIFE INSURANCE
  COMPANY


By
   ----------------------------------
   Its

MELLON BANK, N.A., solely in its capacity as
  Trustee for the Long-Term Investment Trust
  (as directed by John Hancock Life Insurance
  Company), and not in its individual capacity


By
   ----------------------------------
   Its

THE NORTHERN TRUST COMPANY, as Trustee of the
  Lucent Technologies Inc. Master Pension Trust

By:      John Hancock Life Insurance Company,
         As Investment Manager



       By
         ----------------------------
          Its


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<PAGE>   8

         The foregoing Third Amendment Agreement and the amendments referred to
therein are hereby accepted and agreed to as of May 31, 2000, and the
undersigned hereby confirms that on May 31, 2000 it held the aggregate
principal amount of Outstanding Notes of the Company set forth on Schedule 1
hereto and that on the date of execution hereof it continues to hold such
Outstanding Notes.

MASSACHUSETTS MUTUAL LIFE INSURANCE
  COMPANY


By
   ----------------------------------
   Its


                                      -8-
<PAGE>   9

         The foregoing Third Amendment Agreement and the amendments referred to
therein are hereby accepted and agreed to as of May 31, 2000, and the
undersigned hereby confirms that on May 31, 2000 it held the aggregate
principal amount of Outstanding Notes of the Company set forth on Schedule 1
hereto and that on the date of execution hereof it continues to hold such
Outstanding Notes.

PRINCIPAL LIFE INSURANCE COMPANY
(f/k/a Principal Mutual Life Insurance Company)



By
   ----------------------------------
   Its


By
   ----------------------------------
   Its


                                      -9-
<PAGE>   10

         The foregoing Third Amendment Agreement and the amendments referred to
therein are hereby accepted and agreed to as of May 31, 2000, and the
undersigned hereby confirms that on May 31, 2000 it held the aggregate
principal amount of Outstanding Notes of the Company set forth on Schedule 1
hereto and that on the date of execution hereof it continues to hold such
Outstanding Notes.

NEW YORK LIFE INSURANCE COMPANY


By
   ----------------------------------
   Its


NEW YORK LIFE INSURANCE AND ANNUITY
  CORPORATION

By:    New York Life Asset Management
       Operating Company, LLC,
       its Investment Manager



By
   ----------------------------------
   Its


                                     -10-
<PAGE>   11

         The foregoing Third Amendment Agreement and the amendments referred to
therein are hereby accepted and agreed to as of May 31, 2000, and the
undersigned hereby confirms that on May 31, 2000 it held the aggregate
principal amount of Outstanding Notes of the Company set forth on Schedule 1
hereto and that on the date of execution hereof it continues to hold such
Outstanding Notes.

TEACHERS INSURANCE AND ANNUITY
  ASSOCIATION OF AMERICA


By
   ----------------------------------
   Its


                                     -11-
<PAGE>   12

         The foregoing Third Amendment Agreement and the amendments referred to
therein are hereby accepted and agreed to as of May 31, 2000, and the
undersigned hereby confirms that on May 31, 2000 it held the aggregate
principal amount of Outstanding Notes of the Company set forth on Schedule 1
hereto and that on the date of execution hereof it continues to hold such
Outstanding Notes.

KEYPORT LIFE INSURANCE COMPANY

By Stein Roe & Farnham Incorporated, as agent


By
   ----------------------------------
   Its


                                     -12-
<PAGE>   13

         The foregoing Third Amendment Agreement and the amendments referred to
therein are hereby accepted and agreed to as of May 31, 2000, and the
undersigned hereby confirms that on May 31, 2000 it held the aggregate
principal amount of Outstanding Notes of the Company set forth on Schedule 1
hereto and that on the date of execution hereof it continues to hold such
Outstanding Notes.

J. ROMEO & CO.


By
   ----------------------------------
   Its


                                     -13-
<PAGE>   14

         The foregoing Third Amendment Agreement and the amendments referred to
therein are hereby accepted and agreed to as of May 31, 2000, and the
undersigned hereby confirms that on May 31, 2000 it held the aggregate
principal amount of Outstanding Notes of the Company set forth on Schedule 1
hereto and that on the date of execution hereof it continues to hold such
Outstanding Notes.

PACIFIC LIFE INSURANCE COMPANY
(formerly Pacific Mutual Life Insurance Company)


By
   ----------------------------------
   Its


By
   ----------------------------------
   Its


PACIFIC LIFE INSURANCE COMPANY


By
   ----------------------------------
   Its


By
   ----------------------------------
   Its


                                     -14-
<PAGE>   15

         The foregoing Third Amendment Agreement and the amendments referred to
therein are hereby accepted and agreed to as of May 31, 2000, and the
undersigned hereby confirms that on May 31, 2000 it held the aggregate
principal amount of Outstanding Notes of the Company set forth on Schedule 1
hereto and that on the date of execution hereof it continues to hold such
Outstanding Notes.

PHOENIX HOME LIFE MUTUAL INSURANCE
  COMPANY


By
   ----------------------------------
   Its


                                     -15-
<PAGE>   16

         The foregoing Third Amendment Agreement and the amendments referred to
therein are hereby accepted and agreed to as of May 31, 2000, and the
undersigned hereby confirms that on May 31, 2000 it held the aggregate
principal amount of Outstanding Notes of the Company set forth on Schedule 1
hereto and that on the date of execution hereof it continues to hold such
Outstanding Notes.

RELIASTAR LIFE INSURANCE COMPANY


By
   ----------------------------------
   Its


                                     -16-
<PAGE>   17

         The foregoing Third Amendment Agreement and the amendments referred to
therein are hereby accepted and agreed to as of May 31, 2000, and the
undersigned hereby confirms that on May 31, 2000 it held the aggregate
principal amount of Outstanding Notes of the Company set forth on Schedule 1
hereto and that on the date of execution hereof it continues to hold such
Outstanding Notes.

COLUMBIA UNIVERSAL LIFE INSURANCE COMPANY



By
   ----------------------------------
   Its


                                     -17-
<PAGE>   18

         The foregoing Third Amendment Agreement and the amendments referred to
therein are hereby accepted and agreed to as of May 31, 2000, and the
undersigned hereby confirms that on May 31, 2000 it held the aggregate
principal amount of Outstanding Notes of the Company set forth on Schedule 1
hereto and that on the date of execution hereof it continues to hold such
Outstanding Notes.

PROTECTIVE LIFE INSURANCE COMPANY


By
   ----------------------------------
   Its


By
   ----------------------------------
   Its


                                     -18-
<PAGE>   19

         The foregoing Third Amendment Agreement and the amendments referred to
therein are hereby accepted and agreed to as of May 31, 2000, and the
undersigned hereby confirms that on May 31, 2000 it held the aggregate
principal amount of Outstanding Notes of the Company set forth on Schedule 1
hereto and that on the date of execution hereof it continues to hold such
Outstanding Notes.


ALLSTATE LIFE INSURANCE COMPANY


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


By
   ----------------------------------
   Name:
           Authorized Signatories


                                     -19-
<PAGE>   20

         The foregoing Third Amendment Agreement and the amendments referred to
therein are hereby accepted and agreed to as of May 31, 2000, and the
undersigned hereby confirms that on May 31, 2000 it held the aggregate
principal amount of Outstanding Notes of the Company set forth on Schedule 1
hereto and that on the date of execution hereof it continues to hold such
Outstanding Notes.


JEFFERSON PILOT FINANCIAL INSURANCE COMPANY

(FKA Chubb Life Insurance Company of America)



By
   ----------------------------------
   Its




                                     -20-
<PAGE>   21


                                   SCHEDULE 1

<TABLE>
<CAPTION>

                                                                                           PRINCIPAL AMOUNT AND
                                                                                           SERIES OF OUTSTANDING
                  NAME OF HOLDER                                                             NOTES HELD AS OF
               OF OUTSTANDING NOTES                                                            MAY 31, 2000


<S>                                                                                       <C>
John Hancock Life Insurance Company                                                       $13,000,000 1996 Notes

John Hancock Life Insurance Company                                                       $ 8,000,000 1996 Notes

John Hancock Variable Life Insurance Company                                              $ 1,000,000 1996 Notes

Mellon Bank, N.A., Trustee under the Long-Term
  Investment Trust dated October 1, 1996                                                  $   960,000 1996 Notes

The Northern Trust Company, as Trustee of the
  Lucent Technologies Inc. Master Pension Trust                                           $ 2,040,000 1996 Notes

Massachusetts Mutual Life Insurance Company                                               $15,000,000 1996 Notes

Principal Life Insurance Company                                                          $15,000,000 1996 Notes

New York Life Insurance Company                                                           $12,500,000 1996 Notes

Teachers Insurance and Annuity Association of America                                     $12,500,000 1996 Notes

Keyport Life Insurance Company                                                            $10,000,000 1996 Notes

J. Romeo & Co.                                                                            $ 3,500,000 1996 Notes

J. Romeo & Co.                                                                            $ 4,000,000 1996 Notes

Pacific Mutual Life Insurance Company                                                     $ 5,500,000 1996 Notes

Phoenix Home Life Mutual Insurance Company                                                $ 5,000,000 1996 Notes

ReliaStar Life Insurance Company                                                          $ 5,000,000 1996 Notes

Columbia Universal Life Insurance Company                                                 $ 2,000,000 1996 Notes
</TABLE>


<PAGE>   22
<TABLE>
<S>                                                                                   <C>
Allstate Life Insurance Company                                                       $ 2,000,000 1996 Notes

Protective Life Insurance Company                                                     $ 3,000,000 1996 Notes

Pacific Life Insurance Company                                                        $12,000,000 Series A Notes

Pacific Life Insurance Company                                                        $ 8,000,000 Series B Notes

New York Life Insurance Company                                                       $ 5,000,000 Series B Notes

New York Life Insurance and Annuity Corporation                                       $ 7,000,000 Series B Notes

Allstate Life Insurance Company                                                       $ 4,285,714.29 Series C Notes

Chubb Life Insurance Company of America                                               $ 5,000,000 Series D Notes

J. Romeo & Co.                                                                        $ 5,000,000 Series E Notes
</TABLE>