At its most basic, Cover 3 has three deep zones and four underneath zones. The three deep zones are occupied by the free safety (who occupies the middle third) and the two outside corners (who drop into their respective thirds).
Underneath, the strong safety ($) and the open-side linebacker (W) will occupy the two curl/flat zones while the remaining linebackers sink into the two hook zones in the middle. Dropping the extra safety close to the line of scrimmage gives the defense an 8-man box allowing Cover 3 to be a base run defense. The basic Cover 3 looks something like this:
The free safety takes a deep drop and looks to break on the post or dig route or help over the top of an outside 9 route.
The corners, meanwhile, typically use one of two alignments: a soft cushion at the snap (shown above), or a "press-bail" technique, where they line up as if they're going to press the receiver but then "bail" at the snap to create a cushion. Either way, the corners are tasked with covering the receivers vertically and funneling deep, inside-breaking routes to the free safety.
Finally, the middle hook defenders drop to just outside the hash marks. They're tasked with protecting the seam and breaking on underneath crossing routes.
You'll notice that the above image is labeled Cover 3-Sky. The "S" in Sky identifies the force/contain player (the strong safety), just as the "B" in Cover 3-Buzz ('backer) and the "C" in Cover 3-Cloud (cornerback).
Cover 3-Buzz
Below, you can see Pittsburgh's defense in Cover 3-Buzz. Here, the force player is a linebacker rather than the strong safety. The strong safety (Polamalu) is dropping into a hook, and both outside linebackers are responsible for the curl/flat zones (at the top and bottom of the image).
Cover 3-Cloud
In this variation, the cornerback is the force/contain player. In the image below, the Ravens are showing 3-Cloud, and the strong safety is taking one of the deep third zones (bottom of the image). Below him, the corner stays in the curl/flat. One final difference: The hook defenders now must also help with curl responsibility if there's a receiving threat present.
Outside References
2) Stopping the run with a Cover-3 base defense
3) How Earl Thomas and the Seahawks' defense use the Cover 3
4) The Second Level: What You Need to Know Heading into Super Bowl XLVIII
5) Defensive Back Techniques: Cover 3 Pattern Read Examples
6) Film Study: Kaepernick vs. The Patriots
7) How do you beat Cover 3?
8) The Tape Never Lies: Breaking down the Cover 3
9) Cover 3 Alabama 2008 Cutups
10) Monte Kiffin: 3 Deep Coverage
11) Film room takeaways from the 2013-2014 season
