As seen in: Spider-Man 3, The Dark Knight Rises, X-Men: The Last Stand
Right away the trailer establishes Tony Stark's fear of losing Pepper, hinting that their relationship will be tested to the max in this film. This makes perfect sense; the first film is always the origin, the second is off and running, and the third is the one where we have to inexplicably deal with human emotion all of a sudden.
Assuming Robert Downey Jr. counts as human.
In The Dark Knight Rises, Bruce is forced to suddenly address the fact that Alfred refuses to stay and watch him die despite having no problem with it for two damn films, Spider-Man 3 follows the strain between Mary Jane and Peter, and even the third X-Men movie managed to sneak in a three-way love story between Iceman, Rogue, and that Juno chick. Why? Because they're two movies down and they used all the good villains already -- that's why.