Captain America 3 will be more interesting as Robert Downey Jr. is signing for the movie. This indirectly would connects to the Civil War storyline from the comics. With this move, Marvel’s crossover looks more inevitable.
The plot of Captain America is that Stark would go against Steve Roger’s alternate personality, played by Chris Evans. They both would argue on Superhero Registration Act, which forces anyone with superhuman abilities to reveal their identities to the U.S. government and agree to act as a police force for the authorities.
However Rogers being himself disagree with the program and this sparks the initial argument between the two. Roger’s believe that by initiating the program it threatens the civil liberties. The battle begins between the Avengers.
The movie is expected to be release somewhere in 2016.