Cover three has been around for a while. A long, long while. It was the dominant coverage when your father played football. It was probably pretty prevalent before that. It’s still, if not the dominant coverage today, a huge part of the game. Given the popularity and longevity of the scheme there are a lot of ways to run this coverage, with teams able to switch up who covers the deep zones, how many underneath zones there are, who is in those underneath zones, and even where those underneath zones are. That’s all without going into all the blitzes and line stunts possible.
Cover three begins with three deep zone players. They will divide the field into thirds, with their zones beginning somewhere around twelve yards past the line of scrimmage and continuing all the way to the end zone. Deep zone players job, first and foremost, is ensuring that no one catches the ball in their zone. Barring that they can at least make sure they aren’t beat deep, if the ball is in front of them the defender can make the tackle or at least slow the ballcarrier down enough for pursuit to catch up. In traditional cover three the corners will man the outside deep zones, with the free safety taking the middle of the field.
C31 If a corner is tasked with covering a deep third he will align outside the widest wide receiver with his back to the sideline. The corner will be at least five yards off of the line of scrimmage. Depending on matchup, down and distance and his speed he could be aligned a fair bit deeper than that. On the snap the corner will bail, getting depth and reading both the receiver and the quarterback. Unlike cover two the corner is a pass first player in this scheme, only tasked with making run tackles on long runs and broken plays, and even then slowing down the ballcarrier enough for pursuit to catch up is acceptable. As a result coverage skills will be more important in this scheme, while the ability to mug up a wide receiver or tackle a running back in the flats can be sacrificed.
In traditional cover three the deep safety will usually align around twelve yards off the line of scrimmage, at the snap he will begin to shuffle back a bit, keeping his eyes on receivers threatening his zone as well as the quarterback. In essence he is playing centerfield. Cover three deep safeties have to be pure coverage athletes, tall and strong enough to contest jump balls with tight ends and fast enough to run with speedy slot receivers. Usually this is done by the free safety, however the strong safety can and will serve this role on occasion. Like all deep zone players they are tasked with stopping the pass first, only playing the run when entirely sure there is no risk of a pass. Versus the run the deep safety flows towards the side of the run, trying to hit the running back while not allowing the cutback run.
Cover three underneath coverage is a varied beast, with the roles not really limited to the players playing them. For example, the strong safety can easily play either the hook zone, the seam/curl zone, or even the flats. As can both outside linebackers. So, as a result, we are going to go through what is probably the most common version of underneath coverage, and then we will delve into the variations.
In this version of cover three the strong safety and weak linebacker will play the seam/curl zones on either side of the field, in essence serving the same job as an outside linebacker in cover two versus the pass. First they must make sure no receiver is running up the seam, if that happens the player in this zone will carry him up the field. This job description allows the strong safety to play near the line of scrimmage, providing the defense another player to stuff the run. If there is no threat in the seam the zone defender will then look for outside receivers running hook routes or inside receivers running outs. Anything that threatens the area that is outside of the tackle box and underneath the outside deepest zone. Versus the run the seam/curl player will serve as the force defender, no matter what happens he cannot allow the ballcarrier to beat him to the outside. Your force defender should be a strong tackler, capable of taking down a runner one on one in the open field. However, it isn’t the end of the world if your force defender can only slow down the carrier and force him to cut inside. There are ten other guys running towards the ballcarrier from that direction, one of them should be able to bring him down pretty quick. Should the carrier get outside there’s really only the deep zone defender and a lot of open grass between him and the end zone.
The middle and strong side linebackers will play hook zones, responsible for covering any receiver from about the center to an area a few yards outside the tackle to their side. Versus the run the linebacker will have just about the same task he does in cover two, as a rule inside linebacker fundamentals stay pretty consistent. Run to, fill, run away, shuffle over and try to make the tackle. Versus the pass the linebackers will take their two read steps forward, key the guard, then shuffle back into their zone with their "heads on a swivel," looking for crossing receivers as well as following the quarterbacks eyes. Versus outside scrambles the linebackers, particularly the one the quarterback has scrambled away from, are free to go try to make a play on the quarterback, forcing him to throw now (and possibly make a mistake) or take a hit.
Cover three defensive line play has no real consistency. Teams can play their defensive line pretty much any way they please as long as it’s structurally sound. The State of The U did a good basic overview of defensive line play recently. We will not go into the many intricacies of defensive line play here, just know that in basic cover four there are four players rushing the passer. In a four man front this will be the defensive lineman, in a three man front this will be three defensive lineman and (usually) a linebacker.Cloud
Ok, now we are onto the varieties of cover three. The first variety is "cloud", wherein the corner will assume responsibility for the seam/curl zone while either the strong safety takes the corners zone, or the free safety takes the corners zone and the strong safety takes the deep middle. Some varieties of cloud feature the corner pressing the receiver at the line of scrimmage and playing more of a cover two corner type role. This will often be done against a great deep threat, allowing a defense to both rough him up at the line of scrimmage and have a deep safety should he get past the jam.
The free and strong safety can also trade responsibilities, this will make the strong side linebacker play the seam/curl to his side while the will linebacker takes a hook zone.
There is also the zone blitz, the fire zone, the scheme that every commentator is legally required to talk about at least once per football game. Most commonly the zone blitz is run with three underneath defenders, (two seam/curl players and one hook defender), three deep defenders, and five players rushing the passer. The three deep players will have to be secondary players, but other than that a defensive coordinator can plug anyone in just about anywhere. There’s a reason so many teams run zone blitzes with a 3x3 zone coverage behind it, it’s simple, adjustable and versatile. For a more detailed look into zone blitzing check out the excellent
Starting to see why this scheme is so versatile? Provided your secondary can do a bit of everything a defensive coordinator can present all sorts of looks while still basically running the same coverage. That’s not even taking into account the possibility of only rushing three and having three hook defenders, two seam/curl defenders, and the titular three deep zone players. Some teams (mostly just MSU) will even play with three deep defenders and only two underneath defenders as a way of blitzing six while keeping the defense more conservative than cover zero. On some blitz looks defensive lineman even drop back into coverage in the hook or seam/curl zone.
So, what is cover three weak versus? Cover three struggles when teams can spread the field and force the seam/curl defender into two on one situations. Curl/flat is a great example of this. Teams can also run flood concepts, with a fade route, a deep out, and a flat route providing a three on two. Finally there is four verticals, where either the slots beat the seam defender and have a window open under the strong safety or the running back is free to try to get open versus the hook players. Like all zone coverages cover three is weak versus offenses that can recognize it and call concepts that overload the zones. There has also been the spread of run pass options designed specifically to put underneath zone players into situations where they can’t really be right. At the end of the day, though, cover three is a versatile coverage that has been a mainstay of football for generations and isn’t going away anytime soon.