IGN reports that there will be no online multiplayer in Pikmin 3 (just local multiplayer).
"What we've decided to do is focus on the single-player and local multiplayer aspects, which are really fun. But unfortunately, no online multiplayer for Pikmin 3," says Shigeru Miyamoto.
"Since you would have lots of individual, small creatures, the Pikmin, whose every movement and location is going to be really important in the game, it would be very difficult to sync up over an internet connection."