Have you had a chance to visit both (or all three) schools? If not, can you speak with some current students or professors in your program? The good news is that they are all excellent options! I agree with you about Amherst’s open curriculum. It would be wise to confirm whether or not you will have to repeat coursework at Columbia. Information is power in this situation, and it feels from your post as if there is still a lot to learn about your choices.