Tryin to add my contriubtion to theHSA on the tax form. in highlight is says box 12 of the w2....there are 3 sub boxes for 12...12a..12b...12c...which one do i enter?

Enter all amounts shown in box 12 of your W-2 into the box-12 boxes on TurboTax's W-2 form, nowhere else in TurboTax.  TurboTax automatically transfers the code-W amount from its W-2 form to Form 8889.

In the section where you enter your W-2, there is a pull-down menu for each box 12 entry to select the code that matches your W-2.  You may not have all 4 entries on your W-2, that's fine.  Just enter whatever is on your W-2 and select the matching code from the menu.