also: three deep · single-high zone
One safety in the deep middle, two corners on the deep outsides, and four defenders underneath. The most common single-high zone in football.
A zone coverage dividing the deep field into thirds, with a free safety in the middle and the corners taking the outside thirds. Because only three defenders are deep, a fourth is freed to play underneath or in the run fit, which is the trade: better run support than a two-high shell, and four underneath defenders covering five eligible receivers.
three deep, Cover 3 zoneCover 4, Tampa 2, man freeCover 3 carries the same spot-drop against matched split that Cover 4 does. The matched version has the deep defenders relate to receivers rather than to landmarks, which is how modern versions handle four verticals. So a staff saying Cover 3 has, again, told you the shell rather than the rules.
Usually the first coverage a young defense learns, because the rules are simple and the extra run defender matters most at levels where the run dominates.