Very little is known about Pikmin 3: Invasion of the Pikmin Planet, though, from the title, one could get a pretty good idea. The "Distant Planet" will most likely be invaded by, say aliens, or perhaps even Olimar's kin, after learning of the various treasures in Pikmin 2.
Pikmin 3 was just briefly mentioned in an article in "Pulse" in a recent Nintendo Power issue, when answering a question about the Pikmin series.