Last January, director James Cameron revealed he is planning to release Avatar 2 in December 2014, and Avatar 3 in December 2015. It seems that timetable has moved up a bit, since producer Jon Landau revealed that Avatar 2 won't be ready until 2016. Here's an excerpt from the article.

"According to Landau this morning, the first sequel is 'four years away.' So that's 2016 by a literal count, though we could be bendy and assume that Christmas 2015 would also be an option. Expect Avatar 3 a year later, as the performance capture and cinematography would take place back-to-back with the second film."
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We also reported in September that James Cameron wants to shoot Avatar 2 at 60 frames per second, as opposed to the industry standard 24 frames per second. Jon Landau revealed that James Cameron is committed to the higher frame rate.

"Landau promised that Avatar 2 was going to showcase advanced technology, from the 3D camera system used to the CG and performance capture, but also in terms of a higher framerate. This, he says, is the new frontier that he and Cameron are most interested in pushing."