No, no.
You can xor integers as well as ASC values.
The problem is in the expression
Code:
 Mid$(x, l, 1) = (Asc(Mid$(x, l, 1)) Xor Asc(Mid$(y,leny , 1)) Xor Asc(Mid$(z, lenz, 1)))
Look what you are doing: from the y string and the z string you are always taking only ever the last character.
Make it so:
Code:
 Mid$(x, l, 1) = (Asc(Mid$(x, l, 1)) Xor Asc(Mid$(y,l , 1)) Xor Asc(Mid$(z, l, 1)))
l is the loop variable which increments with every step in the for loop. leny and lenz are constantly pointing the last character of y and z.