Three Deep – Invert (“Sky” Force)
Cover 3
The Pre-Snap Read (PSR) is based on the alignment of the Strong Safety (SS) and the Cornerback (C) on the strong side. Teams will typically define the Tight End (Y) as the strong side, however a scouting report will provide this information. If the SS is aligned with less depth than the C, the read is an invert by the SS; for example, the SS is covering the flat, if a receiver is in the flat.
Confirm 3-deep coverage by the alignment of the Free Safety (FS). If the FS is off the hash and favoring the middle, assume that it will be a 3-deep.
Also the QB must be aware of the weak side, if the Weakside Linebacker 
is in a stack (lined-up behind a defensive lineman or end) or walked off the LOS outside the end man on the line, it denotes a soft corner, with him responsible for the weak flat.
talkfootball 