Apparently making the publicity rounds for the DVD/Blu Ray release tomorrow, Christopher Nolan talked to USA Today and had this to say:
...most third acts in Hollywood stink. Look at the disappointing threequels for The Godfather, Superman and the original Batman. The conclusions weren't much better for the more recent Shrek, Spider-Man and Pirates of the Caribbean.And:
"I don't know why they're hard to do," Nolan says. "Maybe there's so much expectation to them. But I wouldn't want to do one if it weren't going to be as good as the first or second. That's not respectful to the fans."
"The worst thing you could do now that you've gotten the plane back in the air is mess up the landing."