Unless you are really into art (which would send you to CA) I’d choose Middlesex out of those three as a boarder, with the limited info you’ve given.
Milton’s admin and teachers did not impress me during Covid and I think it speaks to a deeper problem. Weak admin, disgruntled and uncooperative teachers.
The reasons to go to CA are the art department that is unique among its peer schools and the quirky kid factor. Being IN town is fun but the low boarding population is a negative (in my opinion, not in everyone’s).
The one problem with Middlesex math is that they don’t let you start ahead even if you test ahead/have already taken lower math. At some point, maybe it’s their pre calc, they think that their course is so unique everyone benefits. I think that’s hooey. MX has an amazing writing seminar that all students take.