Seems like we all should get to know this defense as its all I've been hearing Cunningham talk about. I have tried digging up as much info on this formation and thought I'd share what I found.... hopefully you will do the same and we can all learn something from each other.
The 3-3-5 alignment, sometimes known as the "Mustang" or "30-Stack," typically relies on 3 down linemen, 3 linebackers, and 5 defensive backs: 1 free safety, 2 cornerbacks playing off the line, and 2 roving safeties that move around. The safeties take on 2nd receivers or tight ends and have to be able to come up and make tackles, usually outside the box. Generally, when running a 3-3-5 the user is trying to confuse the offense by "exchanging" different pressures in the A, B, and C gaps using the lineman, linebackers, rovers, and free safety while playing mostly zone or sometimes man coverage behind. Teams that run the 3-3-5 generally use it because they are a fast but sometimes smaller unit who wants to cause assignment issues for the opposing offense. Also, a 3-3-5 can be adjusted based on formation to a 4-3, 3-4, 4-4, Goal Line, etc. with the same starting players.
To effectively play the 3-3-5 the "Front 8," especially the lineman and linebackers must be physical, stout, and tough. The lineman must be able to control their assigned gaps, get an effective pass rush, and take out double teams to free up the linebackers to make plays. The 2 outside or "Stud" linebackers must be effective at pressuring, reading and reacting to the play, and play at a low pad level as they will be taking on lineman and fullbacks while the "Mike" must be intelligent in lining up the defense in a counter to the offense's formation as well as flowing to the ball and being able to shed blockers and make plays. The rovers or "Drillers" must be capable of pressuring, dropping back in coverage, being physical, taking on lineman, keeping contain, and coming up and making plays. The cornerbacks must be good in coverage and be physical tacklers. And finally, the free safety is the most versatile athlete on the field as on any given play he can drop in coverage, pressure the quarterback or play one of the "Mike" backers in the 3-4. In addition to this the free safety must be a physical tackler and hitter and generally your best playmaker and smartest athlete.
Here's another link from "Football Scheme Chalk talk"