Brotherhood actually picks up during the ending of AC2, as in for both Ezio's AND Desmond's ending. Both characters do something at the end, and the game just stops there. Basically though, Brotherhood has a lot more of a focus on Desmond himself, while Ezio finishes what he started... At least he tries to.
Revelations picks up MANY years later in Ezio's story, but it picks up right after the ending of Brotherhood for Desmond's story.
AC3 has a new main character, so the past is completely different, but once again, Desmond's story picks up from the moment it left off in Revelations.
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