That's a usual minimum, though some schools may require only two (can't remember). I don't think I've encountered any that require any more than 3, but many/most allow you to submit up to 4-6. I personally have 5 (6 if a prof ever gets around to it) that I can send, and will send the max allowed to each school. If it's less than 5, I'll submit those that "fit" the school the best- Research letters to research schools, volunteer letters to clinical schools, etc.