TV Guide has set the record straight, talking to “Lost” executive producer Damon Lindelof and Maggie Grace herself about the rumors she would play Kitty Pryde in X-Men 3:
Let’s ask her boss, exec producer Damon Lindelof. “She’s on our show,” he told me at press tour. “That movie starts shooting [this month]. I would be an idiot to let her go do a movie in the middle [of our season].” Lindelof insists no one â€” including Grace herself â€” knows how the X3 rumors got started. “The only call she ever made to us was, ‘You’re going to read about this [in the Hollywood trades]; I don’t know what it is, it’s not true. I didn’t read for the part, I wasn’t offered the part, I didn’t accept the part. I’m happy to be on Lost.'”
Instead, Ellen Page will play the role in the third installment.
Source: TV Guide