You used to be able to bring the 3rd QB in if the first 2 got hurt. Here's the rule (from [en.wikipedia.org])

"Teams will be permitted an Active List of 45 players and an Inactive List of eight players for each regular-season and postseason game. Provided, that if a club has two quarterbacks on its 45-player Active List, a third quarterback from its Inactive List is permitted to dress for the game, but if he enters the game during the first three quarters, the other two quarterbacks are thereafter prohibited from playing."


To my surprise that option was eliminated in prior to the 2011 season. (I thought it was still in effect ...)

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AlbaNY_Ram