As you can see the three man front consists of two defensive ends and one defensive tackle.
The main duties of the defensive line are to get past the offensive line to tackle the running back during a rushing play and to tackle (Sack) the quarterback on a passing play. The defensive line is made up of two positions and they are the defensive tackle (DT) and the defensive end (DE).The defensive line also has two basic formations in which they line up, the three man front and the four man front.Please see the Diagram below for how these positions line up at the start of a play in each of the basic formations:
3 Man Front
4 Man Front
As you can see the four man front has an even split with two defensive tackles and two defensive ends.