Generally, amongst casual football fans (there's nothing wrong with that) the thought of zone coverage means that a player is dropping to a zone and defending that land. As long as they secure that small part of the field, they're good, and don't need to worry about anything else.
That, however, is not the case. As evidenced by the cover 3.
I'll talk about it from a 4-3 perspective, then I'll tell you how it equates with the 3-4 with and without the blitz (simply, though. Not going to go into too much detail there.)
The cover 3 generally has a single high safety covering the deep middle of the field. He's responsible from hash to hash (dependent on where the ball is, but we'll assume the ball is in the middle of the field for this)
Generally, you'll see two corners drop. One corner will be responsible from one sideline to the hash. The other will be responsible from the other sideline to the hash. So each is responsible for 1/3 of the deep field.
The corners are reading #2 to their side. The numbering system is from outside in to the center on each side. So the furthest receiver from the center is #1, the next is #2, then #3, then #4. So, for instance...
So the offense lines up in a typical pro set. Split end to one side, flanker/tight end to the other.
To the flanker/TE side, the flanker is lined up furtest outside, he's #1.
The TE is next, he'll be #2.
The back that swings in that direction becomes #3.
To the other side, the Split End is #1 and the back that swings that way is #2.
So again, the corners are keying #2. Which in this example would have the strong side corner keying the tight end and the weak corner keying the back to his side.
So while the strong corner is watching the tight end, if that tight end goes up the seam, he'll have to keep an eye on him, as he's headed towards his responsibility of deep thirds. If he sees the tight end break towards the flats, he's going to shift his attention to #1, to see what route combinations may be occuring. If nothing shows to his zone (deep third) he can scan the rest of the field for any crossing routes. If that's clear, he can help the flat defender with the route combination underneath. If there is any threat to his zone, he must play his zone.
The weak corner does the same, except keying the running back then combination routes with the split end.
A safety will be rolled up to linebacker alignment, most likely the strong safety. He'll cover the curl/flat zone to his side of the field. He's keying #2 then looking to #3. If the TE runs deep, the SS will pass him off to the corner (as discussed above) and he'll turn his attention to the back out of the backfield (who is #3). If the TE does not go vertical, he stays in his curl/flat responsibility.
Now, let's go back to the FS, who has hash to hash responsibility and is the third part of the deep thirds trifecta. His first read is the uncovered offensive linemen. Uncovered means that there is no defensive player lined up head up or to the OL's inside gap. If the OL pass sets, he will get into a zone drop almost straight back. If the OL run blocks, his responsibility is to fill blood alley (my favorite football term. It means he's coming down hill hard and trying to knock someone out).
His key versus pass is #2 to both sides. If #2 doesn't show, he'll can the field for any threats coming to his third of the field. #1 to both sides would be the next logical choice of going deep into that zone.
Now let's talk about the SAM backer. The SAM is the strong side backer and he'll key the TE. He'll take the hook zone, which to give a general description is the area that is in front of the offensive tackle or around there and about 5-10 yards downfield. He's looking up the strong #2 first (TE in this case) and then the strong #3 (RB). He wants to attack any short dump offs in the hook zone from the inside out. Meaning he wants to force the receiver to get pushed to the sideline.
The WILL (weak side backer) has the same job as the strong safety, essentially.
The MIKE will play the hole and help with any routes to the hook zones.
So how does this translate into the 3-4? Well, with no blitz called, it gives you an extra coverage guy. The secondary remains the same.
The SS will take the flats still to the strong side. The SAM will take the curl zone, which is a bit different. Before, the SS had curl/flats. Now the SS would be more concerned with the flats while still paying attention to the curl. The MIKE will align to the strong side of the formation and he'll take the hook zone. The JACK will align to the weak side of the formation and he'll play hook to curl zone. The WILL keeps the same responsibility to his side, playing curl to flats.
With a blitz on from the 3-4 look, it operates the same way as the 4-3 version, except changing jobs. If the WILL blitzes off the edge, the JACK will take the curl/flats, MIKE plays the hole, SAM takes hook, SS takes curl/flats and the secondary remains the same.
If the QB runs a sprint pass (a pass where he's on the run) all the zones shift TO the sprint side. So the backside flats defender might not play as far to the sideline as he generally would. That's a tough throw for any quarterback to make across his body, so he'll probably get more depth and adjust his zone with every step the quarterback makes in the opposite direction.
I want to point out the cover 3's weaknesses. First and foremost, the cover 3 can be picked apart via flat routes (Short routes near the sideline) or routes that are just a bit deeper than the flats, such as a curl. Why? Because the linebacker/SS are reading multiple receivers, and because the corners are dropping from that zone, it's vacated for a moment. Another weakness is the area between the three deep defenders (the area of each of the hashes).
If you have any questions, feel free to ask