Rumours have been around ever since the first Avatar film that 2 and 3 would be filmed back to back and the second being pencilled for release in 2015. Since then however James Cameron has come out to talk about the fourth instalment, quite ambitious considering the second and third haven't even been written yet. He has stated in an interview with Total Film that he is trying to avoid the route that the Matrix Trilogy went down, in that it left too many questions unanswered and failed to deliver the same brilliance as the first one. He has claimed that the second and third instalments of Avatar will be self contained within themselves but also will have a complete story arc over the entire trilogy. All of this based around the same set of characters we have already seen.
Now the fourth film will then concentrate on before the first Avatar, in essence a prequel. Cameron really has got his long term plan sorted, especially since the first one is still being talked about as a change in the vision of cinema. Cameron has also said that Avatar will be the only films he works on from now on and all his effort is going on the next two films, which will be filmed back to back, and then eventually the fourth film which will have new characters and show how the humans and Na'vi became such enemies. Avatar 2 is the only one with a rough release schedule and that is 2015.