Hi! Well, first off, congratulations! You've got AMAZING options there. All three actually sound incredible, so I would go with what your gut said. That said, have you spoken to current students? Professors?
Amherst + Charlottesville are both IDYLLIC places to spend 2 or 3 years (always helps with the writing!). I have a friend who just graduated from UMass and LOVED it--couldn't speak highly enough about the program, faculty, location, etc. I don't know much about the program itself, but their alumni list is quite impressive, and from what I've heard, they're extremely highly regarded, so I wouldn't put too much weight on prestige or P&W rankings. Also, people seem happy there, which is important.
However, like you said, three classes is a lot, and is worth looking further into. Do you know how many students per course? This matters. Are you designing your own syllabus?
Anyway, $20,000 for THREE years is a really, really unique and beautiful opportunity/gift. All a writer could ever hope for is more (funded!) time. So, I see why you're in a bind. But, like I said, can't go wrong. Did you visit all three? Which school "felt" right?