There is much that is currently unknown about this movie's premise, but here's what is known. This movie will follow the continuing adventures of the superpowered group, the X-Men, led by Professor Xavier, and based at his school for gifted students, as they try to foster good will for mutants like themselves, in a world where some people are born with extraordinary powers, and many ordinary humans fear them for it. One likely subplot to be addressed in this movie is the status of Dr. Jean Grey, who experienced something strange in the second movie, something to do with something called Phoenix... (I know a lot more about Phoenix than that, but I'm trying not to spoil the second movie.) Another possible element that may be introduced in this third movie is the idea of the Sentinels, giant mutant-hunting robots constructed to capture and control the "mutant menace" forever.

--Yahoo