According to a report on Eurogamer, a listing for Assassin's Creed III briefly appeared last night on Xbox.com before being hurriedly taken down. Why? Maybe because it had a listing for "Online co-op 2-4 [players]".
Now, it could have been taken down because this information wasn't supposed to go live yet. That happens all the time. It could also have been taken down, though, because it was a mistake. So don't go expecting this to be a feature present when the game launches.
Hope, sure, just don't expect.
For the record, everything revealed about the game so far says almost nothing of its multiplayer modes.
Assassin's Creed 3 has online co-op [Eurogamer]