It's not really in box 3. Box 3 is the gross amount up above. You enter the Medicare on the same screen as the rest of the SSA-1099. Do you get this screen? See my screen shots at the bottom of this one
https://ttlc.intuit.com/community/retirement/discussion/re-you-simply-enter-the-ssa-1099-just-like-y...
Medicare part B go in the 3rd box down.
