I too think three years old is way waaaay too early for anyone (parent or professional) to be making dire predictions about what will and won't work in a child's life. You've tried one preschool so far - it happened to be Montessori but even Montessori's come in many different flavors and the staff who run them can be extremely influential in the experience children will have there. I also think it's too early for a psych to know for sure with limited testing that a child is PG. If *you* see signs in your child that you feel indicate PG, then yes, your child probably is - but I wouldn't trust that from one session with one psychologist.
I especially think 3 is way way way too early to forward-predict behavioral challenges. I do absolutely believe that for kids who truly have behavioral challenges they can and do show up at and before 3. But I've also known a lot of 3 year olds (including my own kids lol) who have behavioral challenges that are situation-dependent sometimes, other times they are simply because the child is 3. You've done exactly what I would have done in this situation re the behaviors - pull him out of this school and try another school after a break. As time goes on, keep watching. If the challenges with behaviors stop suddenly you'll know it was situational re this school; if they continue then do your best to observe them and understand what sets them off, and then move forward to figure out how to help your ds to cope.
I also think it's important to try to remember that it's not easy to tease out what is giftedness vs something else and sometimes it's easy for us as parents to say "oh, he does ___ but that's because he's gifted/bored/not challenged/etc." Sometimes that is the case, but sometimes our kids are acting out due to other reasons too, so it's important to keep an open mind and really observe what's going on so if there *are* other challenges you recognize them early on.
Best wishes,
polarbear
