If you are looking for flight info, expedia might help in getting the best deal (click for multiple destinations).
For itineries while you are there, the local forums will obviously give the best advice.
You really need to ask on the air travel forum and/or the NY/Montreal forums (not this one which is really meant for travel to London and surrounding areas).
This thread will probably be closed soon.
You don't need to do 3 single trips, you can ceratinly buy a package but yes I'd ask on the air travel forum as they're the experts in that side of things.