I will definitely get the third, but maybe for christmas, not right away. I have only played the first game, so I'll probably buy the second game for now. I'll skip Brotherhood and Revelations though, no one seems to think highly of them.
I think they're worth borrowing or renting. They do develop the storyline a bit more.
A lot of people don't think highly of them because they don't introduce anything drastically different to the AC series. That's true, but they're still very fun. If you beat ACII and you're still wanting more, you can't go wrong with Brotherhood. If you still want more after that, play Revelations.
The only reason you should avoid Brotherhood or Revelations is if you find yourself bored of AC2 or Brotherhood. But if you aren't bored of the game(s), they're definitely worth renting/borrowing. Heck, if you're lucky, you might be able to rent them from The Red Box at your local Wal-Mart, gas station, or McDonalds.
I preordered the collector's edition already. I'm kinda disturbed that the PlayStation 3 version will have exclusive content, but oh well.
If you DO play Assassin's Creed Revelations (as well as Brotherhood), there's a bonus mission you can buy. I recommend you just look up a Let's Play of the bonus mission on YouTube, because the bonus mission does advance some of the plot.
