Hmmm. I can't speak directly to your situation, but my letters for med school consisted of one from an undergrad advisor (many years prior) and two from professors at my postbac institution from whom I took low enrollment 400-level science courses, maybe 20 people in the class. It was a real struggle to get that third letter. I had to register for the class and say, yes, I am truly interested in this subject matter, but I need a third med school letter. I then wrote the letter for this prof in mad libs style, where she could fill in my relative rank, adjectives, etc., and do other editing (at least I hope she did).
I am not an expert on SMPs. Going back to take more classes to get an SMP letter seems like overkill, though. I would apply with anatomy, psych, and the surgeon who writes a template letter and see what happens. During the year you're waiting to hear from the SMP, can you be taking an upper level class somewhere?
Just my thoughts. Hope this is somewhat helpful!