Smash vs Cover-3
Smash vs Cover-3 isn't the easy throw it is vs. Cover-2. Here's how the underneath defender changes the read.
Smash vs. cover-3 is harder than smash vs. cover-2. The cover-3 corner is deep (he's got a deep third), and the curl/flat defender takes the hitch. The corner route has to beat the deep third corner over the top.
From the offense
Throw the hitch first. The curl/flat defender is usually a SS or nickel; he's not as quick to the hitch as the cover-2 corner. The hitch is open at 5-6 yards.
If the curl/flat defender squats hard on the hitch, the corner route over the top of the corner's deep third becomes the throw — but it's a tougher read because the corner is bailing.
A bunch alignment helps. Stack the smash receivers and the cover-3 corner has trouble identifying which one is the deep threat.
From the defense
Cover-3 handles smash structurally — corner takes the deep third, curl/flat takes the hitch. The trick is the curl/flat defender's eyes. He has to take the hitch first and then react to anything else.
If the hitch is the only route to his side, he should jump it. The corner route is the corner's deep third.