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.

Thank you that worked for all Alpha, do you know of a formula for Alpha Numeric with the same 3 character contraint?