The letters A-Z 3 times will give you 26^3 = 17576 unique codes.

In cell A1 =ROW()-1 and fill down to A17576
In cell B1 =MOD(A1,26)+65 and fill down to B17576
In cell C1 =INT(MOD(A1/26,26))+65 and fill down to C17576
In cell D1 =INT(MOD(A1/26/26,26))+65 and fill down to D17576
In cell E1 =CHAR(B1)&CHAR(C1)&CHAR(D1) and fill down to E17576

Column E gives you the unique alpha codes. Copy them and paste special to turn into constants. Then you can use them.