Exhibit 99.3
 [WILLIAM BLAIR MEZZANINE CAPITAL FUND III LETTERHEAD]
  
    A.
    
    Prepayment in Full of Note A. On the Closing Date (defined below), Argyle
Security, Inc. (Parent) shall make a capital contribution to the Company in an amount
equal to $8 million from proceeds of the Bridge Notes (as defined below) and the Company
shall use $5.0 million of such funds and the New Note (as defined below), plus other funds,
to prepay in full the principal amount of, and accrued and unpaid Current Interest on, that
certain Amended and Restated Senior Subordinated Promissory Note A dated January 8, 2009
made by the Company payable to the order of the Purchaser. The Holder shall waive the
requirement of any prior notice from the Company in connection with such prepayment and
shall waive any prepayment premium or fee payable in connection with such prepayment,
including without limitation, the prepayment premium set out in Section 2(c) of
Note A. At the closing, all Deferred Interest on Note A shall be converted into a new note
(the New Note). The New Note shall bear interest, computed on the basis of actual days
elapsed in a 360-day year, at the rate of 20% per annum and such interest shall accrue and
be added to the principal balance of the New Note. The New Note shall convert in the
manner and on the terms set forth in clause (C) below.B.
    
    Modification to Interest Rate on Note. The interest rate on the Note shall be
reduced, effective as of the Closing Date, from 11.58% to 10.00%. Additionally, the
requirement that the interest rate under the Note increase on October 1, 2010 from 11.58%
to 15.58% shall be eliminated.C.
    
    Conversion of New Note Into Parent Common Shares. The New Note shall convert
automatically into shares of Parents common stock as of the closing of a Qualified Equity
Offering (as defined below) at an exchange rate per dollar of interest exchanged equal to
the price per share established for the Qualified Equity Offering; provided that, if the
Qualified Equity Offering does not occur by June 30, 2010, the New Note shall automatically
convert into shares of Parents common stock as of June 30, 2010 at an exchange rate equal
to the price per share equal to the volume weighted average sales price per share of the
Parents common stock from trades quoted on the OTC Bulletin Board for the ten (10) trading
days immediately prior to the Closing Date. For purposes hereof, Qualified Equity
Offering means a rights offering of shares of Parents common stock following the Closing
Date or any other private or public placement of shares of the Parents capital stock for
cash.




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D.
    
    Re-Set of Certain Financial Covenants. The financial covenants
under Sections 4.7(b) and (c) of the Purchase Agreement shall be re-set as follows
commencing with the quarter ending December 31, 2009:
 i.
    
    Senior Debt to EBITDA.
 (a)
    
    2.20 to 1.00 for fiscal quarter ending
December 31, 2009,
 (b)
    
    2.20 to 1.00 for fiscal quarter ending
March 31, 2010,(c)
    
    2.97 to 1.00 for fiscal quarter ending June
30, 2010,and(d)
    
    2.20 to 1.00 for each fiscal quarter ending
thereafter.
 ii.
    
    Total Debt to EBITDA.
 (a)
    
    4.68 to 1.00 for fiscal quarter ending
December 31, 2009,(b)
    
    5.78 to 1.00 for fiscal quarter ending
March 31, 2010,(c)
    
    8.25 to 1.00 for fiscal quarter ending June
30, 2010, and(d)
    
    3.85 to 1.00 for each fiscal quarter
thereafter.
 iii.
    
    Fixed Charge Coverage.
 (a)
    
    0.90 to 1.00 for fiscal quarter ending December 31, 2009,(a)
    
    0.90 to 1.00 for fiscal quarter ending
March 31, 2010, and(b)
    
    1.00 to 1.00 for each fiscal quarter
thereafter.
  E.
    
    Consent to Amendment to Senior Debt. The Purchaser agrees to consent
to the Company entering into an amendment to the Loan and Security Agreement pursuant
to clause (c)(i) below.F.
    
    Board Observation Rights. At the closing, Parent and the Company
will enter into an agreement, in form and substance satisfactory to the Purchaser,
granting the Purchaser the right, so long as any obligations under the Purchase
Agreement, the Note or the New Note are outstanding, to have one (1) observer present
at all meetings of the Board of Directors of each of Parent and the Company. The
foregoing board observer right shall become effective as of the conversion date of the
New Note.G.
    
    Consent to Note Modification Agreements to PDI Seller Notes. The
Purchaser agrees to consent to the modifications to the PDI Seller Notes (as defined
below) on terms consistent with the terms set forth in the commitment letter dated the
date hereof between ISI Detention (as defined below) and the holders of the PDI Seller
Notes.H.
    
    Legal Fees. At the closing, the Company will pay (i) all currently
outstanding legal fees and expenses of the Purchasers outside counsel related to
prior matters involving Parent, the Company and the MML Entities (as defined below)
and their affiliates, and (ii) the reasonable fees and expenses of the Purchasers
outside counsel in connection with the preparation, negotiation and closing of the
Amendment and the transactions contemplated by this commitment letter, in each case
whether or not the Amendment closes.




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By:
    
    William Blair Mezzanine Capital Partners III, L.L.C.,
its General Partner
 By:
    /s/ David M. JonesDavid M. JonesManaging Director   By:
    
    /s/ Donald F. Neville

Name: Donald F. Neville 
Title:   Chief Financial Officer