If the titles are accurate, then Avatar 3 might then be called The Seed Bearer, which isn’t a title that means anything in particular now, but might make sense after we see Avatar 2.
Some rumored titles have leaked that give the film the subtitle The Way of Water.
Very little is known about the various sequels, beyond the fact that the second one will primarily take place in the oceans of Pandora.
We do know that Avatar 2 will pick up many years after the first film, with Neytiri and Jake Sully having had children who will become major parts of the story.