M08/19/20 modder, including the custom defensive playbook mod... Did you mean Cover 2 maybe? EA changed the Cloud coverage this year, it doesn't drop deep on corner routes like last year. Cover 3 and 4 are the worst time to throw corners (except for a slow TE) because there is no zone defender underneath to bait with a short route, allowing the deep third corner to sit on the corner route.

The slower TEs still work on corners in Cover 3 but only if you streak the outside WR on that side to force the CB into his deep third rather than sitting and waiting at 10 yards.

Slot streaks/seams against Cover 3 is by far the cheesiest route in the game in my opinion, especially if the QB can throw a bullet that gets to them before the safety playing deep center third, but in-between the LBs or over the OLB's head. That with a horizontal route like a basic In or shallow Slant or shallow Cross is deadly against most defenses. I quit running them because it got to the point where I could spam them over and over by just hot routing any play/formation in the game and score nearly at will, even on All-Madden.