I had 3, plus my MSPE.
1) Letter from my PI (anesthesia) who I worked for for a year.
2) Letter from Peds Pulmonary doc.
3) Letter from dept chair at home institution (probably the least familiar with me, worked with him maybe twice, but did have a meeting with him and I had just finished my anesthesia rotation so he had access to my anesthesia evals).
In retrospect, I would have gotten a letter from my surgery small-group attending, but at the time I applied it would have been almost a year since my rotation and I didn't want a generic-sounding letter. Which is kind of a shame because I think we connected pretty well.
One of the best pieces of advice I would give MS3s is to get letters right after you finish a rotation if you think you had a good connection.
So, overall pretty non-traditional letters as far as not having a surgery or medicine letter, but I did fine. I would go with whichever letters are going to show a deeper understanding of your talent/work ethic/teamwork, not by specialty. Though at least one anesthesia letter is probably a good idea.