Looking forward to 2025
The idea behind having a three man line is you have moved a player off the line of scrimmage so he is harder to block and can move laterally. Teams started to go that route when there were a lot of option offenses in the 70’s. It was effective against the run.
The “new” approach is the 3-3-5, which removes a player from the traditional front seven, and provides a fifth defensive back. They went to that to counter the numerous receivers in spread offenses, they have five DBs to cover 5 potential receivers.
They can play one of the linebackers at or near the line of scrimmage, as essentially a defensive end, so in effect they can have four linemen if they want. It is harder for the offense to anticipate what the defense is going to do, which is probably the key thing. Lots of players can potentially blitz.