It's 'cause of the long console generation.
Generally speaking with new consoles come new franchises, new games proclaiming to be "different." This is, of course, along with the mainstays of the previous generation.
A portion of these franchises got their start about 2005-07'ish. They were the harbingers of the HD generation, and they've been released in a rough 2-year cycle. With this being the fifth/sixth year of the generation, the games are all on their third iteration.
Most of the others are on a much longer franchise cycle, dating back to 2000-2004. They're part of a decade-based revival I've noticed, where the hot properties from the late '90s and early '00s (Mech Warrior, Age of Empires, Deus Ex, Max Payne) are getting a resurgence. Those that aren't "rebooted" or "reimagined" (X-com, anyone?) receive sequels, and a lot of those are threequels.
Hence the threes.
Expect new franchises at next year's E3, along with at least one new console (besides Nintendo's coffee break).