box 12 which code to put in?

All of them.  You need 3 separate lines entered in Box 12 - - Click on Add another when entering box 12.   How to enter Box 12 

12a - CODE W and amount

12b - CODE C and amount

12c - CODE DD and amount

W2 Box 12a - 12d are just the box numbers.  They don't have to be filled in.  But if they have a Code (1 or 2 capital letters) and amount you need to enter it.  Don't confuse the lowercase letters a-d with the uppercase Codes A, B, C, D, DD, etc.  The lowercase a-d are just part of the box label. You don't enter them.  You can enter the Code and Amount in any box 12.