i asked one of the residents at my program, and he said that if they specifically say on their requirements that they want three letters, then send three. If they say at least 3, then send four if you want. I have four letters - one from my home chairman, one from the chairman at each place i did an externship, and then one from a home orthopedist who is well known and spends the most time with us medical students. So, i'm not sure who i would choose to leave out if I am to send just 3.