Cover three is a specific type of zone defense. It is a three-deep and four-under zone. The three-deep zone is handled by the two cornerback positions and free safety position. The safety is referred to as free because the player will wander where needed and serves be the last defensive player in the line. The strong safety covers the curl/flat zone on the strong side and the linebacker has responsibility for coverage of the curl/flat zone on the weak side.
There are various strengths and weaknesses when considering a cover three zone defense. For a strength, it is great when it comes to defending a run and pass offensive play. It also serves to be a good standard defensive zone for a team to fall back on if every player on defense knows their roles and responsibilities well enough. In regards to weaknesses for the cover three zone defense in football, it is often an easily recognizable defensive zone. The offensive team will often be able to realize it is a cover three zone defense and be able to quickly call a different play in order to adapt to it.