It's probably the best quality 3rd party title ever made on Wii and that alone should get it 300-500k from the 'core' crowd. That and a lack of compelling competition.
To exceed 500k Nintendo (and Capcom) need to sell it on it's social play nature. Wii is a social console. People want to play together. This MUST be marketed as a game dad can play with the boys (and Uncle Phil and cousin Kelly via online). They must show it's a fairly simple game to get into a play. And most of all, show it's fun factor. MH3 demo stations in malls would really boost sales tremendously, in fact, make it an enormous success.
Failure to do the above will leave people thinking it's another complicated, single player game and not what they bought a Wii for.
I'd say MH3 could sell anywhere from 1.5m to 5.5m. It all depends on how well Nintendo showcases it.