ok so this is sort of thread hi-jacking but i am in the same situation. i have 3 solid letters from math professor, bio pofessor, and PI i work for full time right now at a medical school. been waiting on a 4th letter from a physician i worked with as an ER tech but he's been MIA for quite some time. I'm thinking just send with 3...?
The general standard for LOR requirements is 2 from science profs and 1 from non-science prof, but check with each school you plan on applying to to make sure you fill their requirements.