Good Introduction to Basic Coverages (smartfootball.com)

...is really a 2-on-1 against the free safety.

The QB is coached to look off the Free Safety and throw to the #2 receiver away from him.  Well-coached safeties are instructed to not come out of the exact middle of the field until the ball is in the air.  Four verticals against Cover 3 is not designed for the home run.  The QB should deliver the ball on a relative line (a la Denard to Roundtree in the spring game) at 15-20 yards downfield; i.e., behind the LBs and in front of the Free Safety.

A properly coached Cover 3 defense will use its LBs to re-route and not give up easy seam throws.  For a 3-3-5 defense, the alignment of the #2 reciever changes who is responsible for this.  If #2 is a TE or Wing, then one of the Stack Backers (Sam, Will, whatever) is responsible for seam elimination (as we like to call it).  If #2 is a wide slot, then either the Spur or Bandit is responsible. 

The objectives are threefold:

1. Take away the quick seam throw by jamming and running with #2.
2. Widen the seam route to the Corner's zone, or outside 1/3.  Do not let #2 cross your face.
3. Get your eyes to #1.

Once the jam and re-route is accomplished, the LB will key #1.  If #1 continues vertically up the field, the LB will continue to run with #2.  If #1 throttles down, the LB will come off #2 and get to his zone.  The Stack Backer will hunt a crossing route by #1 while the Spur/Bandit will hunt inside-out; i.e., curl to deep out to quick out.  This is an easy read but hard to get to quickly.  The Spur/Bandit must be a superior athlete.  (This is one of my reservations about Kovacs.  I don't believe he can take away a seam and be able to get to a curl against quality QB/WR combos.)

The properly-coached Corner in Cover 3 will align himself on the outside shoulder of #1 at a depth of 7-8 yards (assuming he is not using a bail technique, something I loved to use).  His keys are #2 to #1.  Against 4-verts, the corner will read the vertical release of #2 and pedal with the idea of splitting the distance b/w #1 and #2, and then key #1.  Seeing #1 also vertical, he indeed splits the horizontal distance of #1 and #2 while staying deeper than the deepest.  (If #1 were to run an underneath route, the Corner would then adjust his backpedal to the outside shoulder of #2 without going inside the hash.)

The Mike is responsible for crossing routes b/w the hashes and checkdowns.  Digs and hooks he will attack; shallow crosses and checkdowns he will not attack until the ball is thrown.  Saban rule: in Cover 3, never break on any route under 5 yards until the ball is thrown.  If you do, he will oversign your ass out of town.  (Okay, I added that last part.)

Of course, all this is nice clinic talk.  In reality, a good QB/WR unit can carve up even a good defense in Cover 3 with Four Verticals.

-- oh wait, the other guy already answered this?  OK then, carry on...  Let me know if his explanation wasn't detailed enough, and I can try to fill in some other... football stuff.  I think he may have covered it well enough though