James Cameron has revealed to EW a working release date for Avatar 2 and Avatar 3. He says they will be aiming for Christmas 2014 release for Avatar 2 and the third film the following year. He goes on to mention he is currently writing and again confirms the films will be shot back to back. I assume the three films could end up being seamlessly interweave together similar to what was done with Lord of The Rings. The films are expected to spend time exploring the rest of Pandora including it's oceans.
"I am in the process of writing the next two Avatar films now. We are planning to shoot them together and post them together, and we will probably release them not quite back to back, but about a year apart. Christmas ’14 and ’15 is the current plan."