They're all elite LACs in the northeast... Preppy is inevitable at all three. With that being said, Hamilton has a pretty outdoorsy vibe (at least when i visited). However, there are still lots of preppy kids (which was a good thing for me). I think Williams would probably be the most preppy of these 3. I'm not sure about Midd, but from what I've heard it's probably in between Hamilton and Williams.
They're all preppy. If you want to avoid preppy, you need to look at the midwestern LACs and some of those in the NW. If you can handle some prep, but in smaller doses, the mid-Atlantic LACs might work, as would the Claremont consortium in CA.