Very few actually do that. Off the top of my head, only GWU and Tennesse do that I think, most schools take your latest MCAT score regardless of which one is the highest. And a few take the average composite scores (so if you took the MCAT and got a 28 and then a 34, the medical school will average to be 32).
Don't know - I guess they don't want you to be taking it 5 times in hopes of getting lucky. The schools that average your scores are few though. Most schools simply take your latest MCAT score unless they say otherwise.
Are you asking which schools cherry pick the best section scores or best overall score from a given test? For example having a 3/14/5 one time, then a 14/3/6, and then a 3/5/14 the last time to give a score of 42? Or taking the best overall score from each of these which would be 23? Yes, I realize this is an absurd example. I think GWU uses the first method described here, whereas many more medical schools will probably use the second method. I've never heard of a school averaging MCAT scores.