Things work better in threes.
Three factions > two factions. I only ever experienced this very briefly in DAOC, and you could say I didn’t experience it at all since I didn’t RvR, but it still makes sense to me. One just doesn’t see it a lot in games, probably because a) we’re used to thinking in twos, even designers, and b) it’s hard enough designing two factions without having to come up with a third. That said, I could come up with a third faction in WoW pretty easily: extract all the self-righteous lawful-good types and there’s the Alliance; extract all the extra-mean evil types and there’s the Horde; and everyone else is your third faction – droods, worgen, goblins, whatever.
Three hands > two. I am not able to vouch for this personally, but think how useful it could be to have two hands for holding the shopping and still have one hand with which to unlock the house door! One hand to mouse, one hand to key, and one hand to give that #(*&@(* who just killed you the finger. I don’t PvP but that might come in handy, no?
If two heads are better than one, then surely three heads > two.
Three MMO genres I have yet to experience: Steampunk, Cyberpunk, Western.
Three blind mice.
Three French hens, though I’m not entirely convinced about this one.
Tank. Healer. DPS. (Yeah… we’ll come back to this one someday.)
P.S. – I am cheating and posting this to yesterday’s date. I have my reasons.