I would recommend Revelations. A lot of interesting things happen and they sum up brotherhood pretty well. Brotherhood is fun but not needed for story. --- Miyamoto Meter 9 out of 10 "E3 Legend of Zelda Cosplay"
Nothing in the over arching plot happens in Revelations, but skipping Brotherhood will leave you with some questions, namely "Where is Lucy?" and "How did Desmond get an Apple of Eden?" which are very plot relevant. --- R.I.P. Junk Bludgeo'd For Hammer, But Always In Our Hearts. 176 Damage, Nevar Forget.
It would definitely help to play Brotherhood and Revelations. Although the intro does help you get up to speed, some of the characters talk about some really spoilerific moments from those games (And the Lost Archives DLC for Revelations).
You should definitely play Brotherhood, Revelations is skippable if you really want to hurry on to 3, but I still recommend it for if you want to know how Desmond got out of the jam he found himself in at the end of Brotherhood, and if you want to watch the conclusion of the Subject 16 story. --- I came here to chew bubblegum and kick ass, and I'm all out of bubblegum.
Anyone who says that you NEED to play them is full of it. You're really only missing out on 10% of the game if you come into ACIII blind to the whole series. Most of the game (the stuff that takes place in the past) does NOT reference the other games and is it's own self contained story. The stuff that takes place in the present only happens in 20 minute segments and is really easy to understand. It's like jumping into the middle of any franchise, yeah whatever you don't understand the specifics but you get the gist of what's going on and can enjoy it. There's also a crapload of database entries that pop up every 2.4 seconds to expand on anything you miss. --- this topic is ruined [i really hate classic rock]