The way I did it was to get 1 LOR from my PI, a LOR from 2 science/math profs, and 1 humanities prof. While post-docs might know you better, I think med schools want letter of recs from actual professors.
Your letter of rec from your PI may count the 2 LOR from science prof requirement because your PI is probably some sort of professor (assistant/associate if not full). I think you will still need to get a LOR from a science professor with whom you took a class. If you're still a little early in the application cycle, I would suggest taking a seminar course as these tend to be smaller and you can get to know your professor better.
In the case of some schools requiring 2 lab recs...some schools are lenient on this policy if you've stayed with the same lab for multiple years and will only require you to get a LOR from your PI. Just check with the school to make sure.