How should degrees be listed in an e-mail signature?
There is no formal academic convention for email signatures, although your university or institution may have formatting guidelines. You can simply include as much or as little information as you want the recipient to know.

Personally, I think your name and position are sufficient and listing every degree you have is a bit redundant (and, as others have pointed out, pretentious). People will probably infer that you have a BS and MS if you also have a PhD.