Well, from what I can ascertain, they'll probably be using the "Extremis" storyline as a basis for the plot, where Tony essentially becomes one with the Iron Man armor, and they are going to be introducing Iron Man's long-time arch enemy, the Mandarin, played by Ben Kingsley.
For more info on the Mandarin, here's a link:
http://en.wikipedia....ndarin_(comics)
And here's some info about the "Extremis" story arc:
http://en.wikipedia....tremis_(comics)
Also, IM3 is supposed to be the first chapter of "Phase 2" of the Marvel Cinematic Universe, all of which will be leading up to the next Avengers movie.
FUN FACT: The "Ten Rings" terrorist group in IM1 is a reference to the Mandarin's Ten Rings of Power, and it is VERY likely that the group was connected to the villain.