James Cameron has commented in a recent interview that he plans to shoot the next two parts of "Avatar" jointly:
“We are now talking about shooting the two sequels one after the other. It is not yet a firm decision but it is something that makes a lot of sense given the nature of these productions.
The reason these back-to-back movies (shot simultaneously) end up being canceled is because you try to make two live action movies at the same time and work on it for a year and a half, ending the whole world exhausted. It is not humanly possible. For this type of film that works perfectly. "
With this, "Avatar 2" would be released in theaters in 2014 and, possibly, "Avatar 3" would do so in 2015.