I'm a med student at Dalhousie. There are a couple of Americans in my class right now. Your chances at Dal as an american are equal to someone from outside of the Maritime provinces (Dal divides us into Maritime, and non-maritime students). The non-maritime spots are quite competative though. (Maritime =Nova Scotia, Prince Edward Island and New Brunswick.)
Memorial has about 15 spots reserved for Americans. I don't know much about McGill, though I know lots of Americans do go there. Do you speak french? It would really help if you want to live in Montreal.
Goodluck.