I am familiar with all three schools. They are all great schools and I would recommend all of them (in general – obviously it depends on the person and I don’t know you). They all have very warm atmospheres and are very growth-oriented. MMY is much more academic than the others and you have to be prepared to work hard. I know that MMY gets really good girls (or at least they used to, and I am assuming it is still the case). I don’t know about the other two. I would assume that it’s true of the others as well, but I don’t know enough.
Writer-soul did a great job of describing the schools. I would just add a few corrections, with the qualification that my information may not be completely up-to-date, but I am pretty sure that it is still fairly accurate. For one thing, I would not describe MMY as completely MO. They have teachers with different hashkafos and they do not push a specific hashkafa. Most of the teachers are Dati-Leumi but there are some who are Chareidi, and there are students who have become Chareidi there. However, it is rare for a student from a Bais Yaakov high school to go there.
Last I heard, MMY was an extremely warm place, and it is hard for me to imagine that has changed.
One major difference between Shaalvim and MMY is that Shaalvim does not have ANY Chareidi teachers, and MMY does. And that is updated information since it is something that I happened to have looked into recently. Also, if you check out Shaalvim’s website, it is EXTREMELY tzioni and seems to be much more definitively MO than MMY. (I actually would not have used the word MO to describe MMY – I would have described it as closer to Chardal, but I have the impression that the term MO might be used differently today than it used to be).