<PAGE>
 
                                                            Exhibit No. 10(a)(3)

                      THIRD AMENDMENT TO CREDIT AGREEMENT


       THIS THIRD AMENDMENT TO CREDIT AGREEMENT,  dated as of June 13, 1997
(this "Amendment"), is entered into by and among ICF KAISER INTERNATIONAL, INC.
("Borrower"), a Delaware corporation, each of its subsidiaries signatories
hereto (each a "Subsidiary Guarantor" and collectively the "Subsidiary
Guarantors"), the banking institutions signatories hereto (each, a "Bank" and
collectively, the "Banks") and CORESTATES BANK, N.A., as agent for the Banks
under this Agreement (in such capacity, the "Agent").

                                   WITNESSETH
                                   ----------

       WHEREAS, Borrower, each Subsidiary Guarantor, the Banks and the Agent are
parties to a Credit Agreement, dated as of May 6, 1996, as amended by the First
Amendment dated as of December 17, 1996, and the Second Amendment dated as of
May 5, 1997 (the "Credit Agreement"), whereby the Banks have agreed to provide a
revolving credit facility for loans and for letters of credit;

       WHEREAS, the Borrower and the Subsidiary Guarantors have requested, and
the Banks and the Agent have agreed, to amend the Credit Agreement in certain
respects, as provided herein.

       NOW, THEREFORE, in consideration of the premises and intending to be
legally bound hereby, the parties hereto agree as follows:

1.     Amendment to Credit Agreement

       a.   The following definitions are hereby added to Section 1.1:

            "Hunters Branch Investment" shall mean the Investment by the
       Borrower or any Subsidiary in the buildings and associated land at 9300
       and/or 9302 Lee Highway, Fairfax, Virginia.

            "ICF Kaiser Hunters Branch" shall mean ICF Kaiser Hunters Branch,
       Inc., an entity (however denominated) organized under the laws of the
       State of Delaware, created in order to own the Hunters Branch Investment.

       b.   Section 7.6 is hereby amended by the addition of the following new
subsection (h) at the end of such Section:

       (h)  Borrower or any Subsidiary may make and own the Hunters Branch
       Investment, provided that the cash amount of such Hunters Branch
       Investment at the closing thereon (excluding transaction costs) shall be
       limited to a maximum of $1,500,000, which amount (when used) shall be
       deducted from the $5,000,000 aggregate Acquisitions limit set forth in
       subsection (d) of this Section 7.6; provided, further, that the Borrower
                                           -----------------
       or any Subsidiary may, but is not required to, capitalize any and all
       transaction costs related to the Hunters branch Investment without
       deducting such capitalized costs from the afore-mentioned Acquisitions
       limit; and provided, further, that ICF Kaiser Hunters Branch may make 
                  -----------------          
       such additional Hunters Branch Investments as are set forth below without
       deducting any such Investments from the afore-mentioned Acquisitions
       limit:

            $600,000 in each of years 1997, 1998, and 1999; and
<PAGE>
 
            $700,000 in each of years 2000, 2001, 2002, 2003, 2004, and 2005.

2.     Conditions Precedent.  The Amendment to the Credit Agreement contained in
       ---------------------                                                    
Section 1 hereof shall be effective upon satisfaction of the following
conditions precedent.

       (a)  Evidence of Authorization.  The Banks shall have received copies
certified by the Secretary or Assistant Secretary of Borrower and each
Subsidiary Guarantor of all corporate or other action taken by such party to
authorize its execution and delivery and performance of this Amendment, the
Second Amendment to the Security Agreement, and the Loan Documents as amended
hereby, together with such other related papers as the Banks shall reasonably
require;

       (b)  Documents.  The Agent shall have received all certificates,
instruments and other documents then required to be delivered pursuant to any
Loan Documents, in each instance in form and substance reasonably satisfactory
to the Agent and the Banks;

       (c)  Other Agreements.  Borrower and each Subsidiary Guarantor shall have
executed and delivered each other Loan Document required hereunder;

3.     Representations and Warranties.
       -------------------------------

       (a)  The Borrower confirms the accuracy of the representations and
warranties made in Article 3 of the Credit Agreement as of the date originally
given and restates to the Banks such representations and warranties, as
previously amended, on and as of the date hereof as if originally given on such
date.

       (b)  The Borrower confirms that as of the date of this Third Amendment,
there has been no litigation, administrative proceeding, investigation, business
development, or change in financial condition which could reasonably be expected
to have a material adverse effect on the business, operations, assets or
condition (financial or otherwise) of the Borrower or its Subsidiaries taken as
a whole.

4.     Covenants.
       ----------

       (a)  The Borrower warrants to the Banks that the Borrower is in
compliance and have complied with all covenants, agreements and conditions in
each Loan Document on and as of the date hereof, that no Potential Default or
Event of Default has occurred and is continuing on the date hereof and that,
upon the consummation of the transactions contemplated hereby, no Potential
Default or Event of Default shall have occurred and be continuing.

       (b)  The Borrowers shall provide to the Agent and its representatives all
requested access and assistance as shall be reasonably necessary for such due
diligence review as the Agent shall determine is necessary or advisable,
including without limitation a collateral audit.

5.     Effect of Agreement.
       --------------------

       This Agreement amends the Loan Documents only to the extent and in the
manner herein set forth, and in all other respects the Loan Documents are
ratified and confirmed.

6.     Counterparts.
       -------------

                                       2
<PAGE>
 
       This Agreement may be signed in any number of counterparts, each of which
shall be an original, with the same  effect as if the signatures hereto were 
upon the same instrument.

7.     Governing Law.
       --------------

       This Agreement and all rights and obligations of the parties hereunder
shall be governed by and be construed and enforced in accordance with the laws
of Pennsylvania without regard to principles of conflict of law.

       IN WITNESS WHEREOF, Borrower and the Banks have caused this Agreement to
be executed by their proper corporate officers thereunto duly authorized as of
the day and year first above written.


     CORESTATES BANK, N.A.                      ICF KAISER INTERNATIONAL, INC. 


     By:    /s/ John D. Brady                   By:    /s/ Michael K. Goldman  
        --------------------------------           ----------------------------
     Name:  John D. Brady                       Name:  Michael K. Goldman
     Title: Assistant Vice President            Title: Executive Vice President

     BHF-BANK AKTIENGESELLSCHAFT                SIGNET BANK


     By:    /s/ Linda Pace                      By:    /s/ Brian Haggerty      
        --------------------------------           ----------------------------
     Name:  Linda Pace                          Name:  Brian Haggerty          
     Title: Vice President                      Title: Vice President

     By:    /s/ John Sykes
        --------------------------------
     Name:  John Sykes
     Title: Assistant Vice President

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<PAGE>
 
<TABLE> 
<S>                                           <C>                                          <C> 

The Subsidiary Guarantors:

CLEMENT INTERNATIONAL                         CYGNA GROUP, INC.                            HENRY J. KAISER COMPANY
CORPORATION.


By:/s/ Timothy P. O'Connor                    By:/s/ Timothy P. O'Connor                   By:/s/ Timothy P. O'Connor
   -----------------------                       -----------------------                      -----------------------         
Name:  Timothy P. O'Connor                    Name:  Timothy P. O'Connor                   Name:  Timothy P. O'Connor    
Title: Assistant Treasurer                    Title: Assistant Treasurer                   Title: Assistant Treasurer    
                                                                            

EXCELL DEVELOPMENT                            ICF INFORMATION                              ICF INCORPORATED
CONSTRUCTION, INC.                            TECHNOLOGY, INC.


By:/s/ Timothy P. O'Connor                    By:/s/ Timothy P. O'Connor                   By:/s/ Timothy P. O'Connor 
   -----------------------                       -----------------------                      -----------------------
Name:  Timothy P. O'Connor                    Name: Timothy P. O'Connor                    Name:  Timothy P. O'Connor
Title: Assistant Treasurer                    Title: Assistant Treasurer                   Title: Assistant Treasurer
                                                                                           
ICF KAISER ENGINEERS                          ICF KAISER ENGINEERS                         ICF KAISER ENGINEERS 
CORPORATION                                   (CALIFORNIA)                                 MASSACHUSETTS, INC. 
                                              CORPORATION


By:/s/ Timothy P. O'Connor                    By:/s/ Timothy P. O'Connor                   By:/s/ Timothy P. O'Connor
   -----------------------                       -----------------------                      ----------------------- 
Name:  Timothy P. O'Connor                    Name:  Timothy P. O'Connor                   Name:  Timothy P. O'Connor
Title: Assistant Treasurer                    Title: Assistant Treasurer                   Title: Assistant Treasurer
                                                                          
ICF KAISER ENGINEERS                          ICF KAISER GOVERNMENT                        ICF KAISER ENGINEERS, 
GROUP, INC.                                   PROGRAMS, INC.                               INC.  
                                                                          
                                                                          
By:/s/ Timothy P. O'Connor                    By:/s/ Timothy P. O'Connor                   By:/s/ Timothy P. O'Connor
   -----------------------                      -----------------------                       -----------------------  
Name:  Timothy P. O'Connor                    Name: Timothy P. O'Connor                    Name:  Timothy P. O'Connor
Title: Assistant Treasurer                    Title: Assistant Treasurer                   Title: Assistant Treasurer
                                                                          
ICF KAISER HOLDINGS                           ICF KAISER HANFORD                           ICF RESOURCES 
UNLIMITED, INC.                               COMPANY                                      INCORPORATED 

By:/s/ Timothy P. O'Connor                    By:/s/ Paul Weeks, II                        By:/s/ Timothy P. O'Connor
   -----------------------                      -----------------------                       -----------------------   
Name:  Timothy P. O'Connor                    Name:  Paul Weeks, II                        Name:  Timothy P. O'Connor
Title: Assistant Treasurer                    Title: Assistant Secretary                   Title: Assistant Treasurer
</TABLE> 

                                       4
<PAGE>
 
<TABLE> 
<S>                                           <C>                                          <C> 

ICF LEASING                                   KE SERVICES                                  KE LIVERMORE, INC.
CORPORATION, INC.                             CORPORATION


By:/s/ Timothy P. O'Connor                    By:/s/ Timothy P. O'Connor                   By:/s/ Timothy P. O'Connor    
   -----------------------                       -----------------------                      -----------------------    
Name:  Timothy P. O'Connor                    Name:  Timothy P. O'Connor                   Name:  Timothy P. O'Connor 
Title: Assistant Treasurer                    Title: Assistant Treasurer                   Title: Assistant Treasurer 
                                                                                                                          

KAISER ENGINEERS AND                          KAISER ENGINEERS                             CYGNA CONSULTING 
CONSTRUCTORS, INC.                            INTERNATIONAL, INC.                          ENGINEERS & PROJECT 
                                                                                           MANAGEMENT, INC. 


By:/s/ Timothy P. O'Connor                    By:/s/ Timothy P. O'Connor                   By:/s/ Timothy P. O'Connor
   -----------------------                       -----------------------                      -----------------------     
Name:  Timothy P. O'Connor                    Name:  Timothy P. O'Connor                   Name:  Timothy P. O'Connor
Title: Assistant Treasurer                    Title: Assistant Treasurer                   Title: Assistant Treasurer

TUDOR ENGINEERING                             PCI OPERATING COMPANY,                       SYSTEMS APPLICATIONS 
COMPANY                                       INC.                                         INTERNATIONAL, INC. 


By:/s/ Timothy P. O'Connor                    By:/s/ Timothy P. O'Connor                   By:/s/ Timothy P. O'Connor
   -----------------------                       -----------------------                      -----------------------     
Name:  Timothy P. O'Connor                    Name:  Timothy P. O'Connor                    Name:  Timothy P. O'Connor      
Title: Assistant Treasurer                    Title: Assistant Treasurer                    Title: Assistant Treasurer       

</TABLE> 

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