One of my friends told me that you get special unlocks in Modern Warfare 3 if you went prestige in the first two.
Is this true? If so what sort of unlocks are we talking about?
Modern Warfare 3 features the Prestige Shop. Prestige you've performed in a previous Call of Duty games (MW1, World at War, MW2, Black Ops) earns you a single prestige token per game.
Prestige tokens can be spent in the shop to unlock things like an additional custom classes, special emblems and titles, and numerous other features.
Full list (from here):