In my experience, (7 years of regular presence in Montessori classrooms) most 3 yr. olds are NOT doing the 100 board. I would say that because the classrooms are multi-age that it is likely if the teacher says your child is advanced she has seen enough typically developing children to know what she is talking about and I would for sure begin exploring options now to prepare for the future.
FWIW Montessori has been wonderful for my now dd11, and I know that she would have been bored, frustrated and unchallenged at our local public school. I think it depends on the flexibility of the school, the individual teachers, and the kid. There are many ways to extend the classroom materials (you can look on youtube to see the more advanced things they can do with the stairs and rods etc.) that could possibly keep your child challenged. And for a kid who might have a hard time sitting still at a desk a Montessori environment is far more accomodating.
So far it has been a good fit for my dd4, but I don't know if that would continue to be the case b/c her temperment/personality is so different than dd11. We are moving and she will be starting in public K in the fall.
Good luck with your decision.
