Cover-3 is a very common defensive concept among NFL teams. It is often used to prevent big plays down field without compromising too much of the coverage underneath. It seemed to be a staple of former Bengals defensive coordinator Paul Guenther. In fact, Pro Football Focus shows the Bengals were the most successful team at running that coverage last season.
Here’s PFF’s explanation of Cover-3:
Cover-3 splits the field into deep thirds, with the free safety taking the middle, and the outside cornerbacks dropping deep on either side. This allows the defense to put eight men in the box using the strong safety, so is something we often see on earlier downs.