I would really wait and see. Your son is only 3 years old, right? How can anyone really say that "no school will ever work." Who knows in a 3 year old? Maybe your son needs to mature a little more; maybe at 4 or 5 or 6, he can follow more rules in a group, etc.
For what it's worth- Montessori schools tend to be very unstructured. Maybe your son will do alot better with more structure- I don't mean telling him rigidly what to do. What I mean is- when my son was in a "real" Montessori, they wanted him in first grade to go in, make a list of things he would do, then do them. Well, he never did that! He wandered around the classroom, ultimately doing some math, and wasting the whole day.
In a good public school where they have some kind of structure, it's been alot better.
