Apparently, the studio has wanted the actress for quite a while, but talks continued to breakdown because of scheduling conflicts.
Chastain was preoccupied with two other films, including Kathryn Bigelow's Osama Bin Laden drama.
Thankfully, Chastain's other gigs have been moved around giving her some much-needed free time.
Both Paltrow and Cheadle will be reprising their roles as Virginia "Pepper" Potts and James "War Machine" Rhodes, while newbies Kingsley and Pearce will play the film's villains.
Iron Man 3 will center on the spread of a fatal virus through nanotechnology, which would explain the need for a new scientist.
Iron Man 3 will be directed by Shane Black and opens May 3, 2013.
And you'll see personalized content just for you whenever you click the My Feed .
SheKnows is making some changes!