A defensive strategy in which three defensive players will play a deep zone across the field in order to deny the opposing team a deep throw. A Cover 3 can be played as a man or a zone: in a Cover 3 man the other players will each guard a player underneath and rely on the deep players to take away a deep route, and in a Cover 3 zone the other players will be responsible for an area near the line of scrimmage.
A Cover 3 is used primarily against teams that are particularly good at completing deep passes. By having a third player deep, it takes away the lanes in which most deep routes would be effective. The downside to a Cover 3 is that it leaves less defenders to guard against the underneath routes, allowing an offense to take short routes and gain yardage slowly. Typically, the Cover 3 is used against explosive passing attacks, or when a team is up near the end of a game and wants to guard against a big play.