<PAGE>   1


                                                                Exhibit 10.1a(3)


                       THIRD AMENDMENT TO CREDIT AGREEMENT


        THIS THIRD AMENDMENT TO CREDIT AGREEMENT (this "Amendment"), dated as of
March 15, 2001, by and among STEEL DYNAMICS, INC., an Indiana corporation (the
"Borrower"), the lenders listed on the signature pages hereof and MELLON BANK,
N.A., a national banking association, as agent for the Lenders under the Credit
Agreement (Amended and Restated) referred to below (the "Agent").


                                    RECITALS:

        WHEREAS the Borrower, certain lenders, the Agent, Mellon Bank, N.A., as
Issuing Bank, and certain Co-Agents entered into a Credit Agreement (Amended and
Restated), dated as of June 30, 1994 and amended and restated as of June 30,
1997 and further amended as of May 4, 1998 and as of March 1, 2000 (as so
restated and amended, the "Original Agreement"), pursuant to which the Lenders
have extended credit to the Borrower;

        WHEREAS, the Borrower and the Required Lenders (as defined in the
Original Agreement) desire to amend the Original Agreement to make certain
changes therein;

        WHEREAS, capitalized terms not otherwise defined herein shall have the
meanings assigned thereto in the Original Agreement.

        NOW, THEREFORE, the parties hereto, in consideration of their mutual
covenants and agreements hereinafter set forth and intending to be legally bound
hereby agree as follows:

        Section 1. Amendment. Section 6.04 of the Original Agreement is hereby
amended by deleting the period at the end of paragraph (g) thereof, by inserting
a semicolon in lieu of such period and adding, at the end of Section 6.04, a new
paragraph (h) to read as follows:

                                (h) the guaranty by the Borrower of not more
                        than $8,250,000 principal amount of Indebtedness of IDI
                        owing to lenders under the Credit Agreement, dated as of
                        December 31, 1997, as amended from time to time, among
                        IDI, certain



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                        Lenders and Mellon Bank, N. A., as Issuing Bank and
                        Agent.


        Section 2. Directions to Agent. By execution of this Amendment, the
Required Lenders hereby direct the Agent to execute and deliver this Amendment.

        Section 3. Miscellaneous. (a) This Amendment shall become effective upon
execution and delivery hereof by the Required Lenders, the Borrower and the
Agent

        (b) The Original Agreement, as amended by this Amendment, is in all
respects ratified, approved and confirmed and shall, as so amended, remain in
full force and effect. From and after the date hereof, all references to the
"Agreement" in the Original Agreement and in the other Loan Documents shall be
deemed to be references to the Original Agreement as amended by this Amendment.

        (c) This Amendment shall be deemed to be a contract under the laws of
the State of New York and for all purposes shall be governed by and construed
and enforced in accordance with the laws of said State.

        (d) This Amendment may be executed in any number of counterparts and by
the different parties hereto on separate counterparts, each of which, when so
executed, shall be deemed an original, but all such counterparts shall
constitute but one and the same instrument.



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        IN WITNESS WHEREOF, the parties hereto, by their officers thereunto duly
authorized, have executed and delivered this Amendment as of the date first
above written.


                                        STEEL DYNAMICS, INC.



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




                                        MELLON BANK, N.A., as Lender
                                          and as Agent


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




                                        KREDITANSTALT FUR WIEDERAUFBAU


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


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



                                        COMERICA BANK



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



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                                        THE INDUSTRIAL BANK OF JAPAN,
                                          LIMITED



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



                                        BANK ONE, INDIANA



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



                                        WACHOVIA BANK, N. A.



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



                                        BANK AUSTRIA AKTIENGESELLSCHAFT



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


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



                                        NATIONAL CITY BANK, INDIANA



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



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                                        LASALLE BANK NATIONAL ASSOCIATION



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



                                        THE CHASE MANHATTAN BANK



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



                                        HARRIS TRUST AND SAVINGS BANK



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



                                        WESTDEUTSCHE LANDESBANK
                                          GIROZENTRALE, NEW YORK BRANCH



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


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



                                        SUNTRUST BANK



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



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                                        NORTHERN TRUST COMPANY



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



                                        FIRST UNION NATIONAL BANK



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



                                        THE HUNTINGTON NATIONAL BANK



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