Three Technique is a term thrown around be the football "experts" without much explanation.  The three technique is one of the defensive tackles in a 4-3 defense.   The word "technique" doesn't refer to what a defensive lineman does, but rather to where he lines up.  I'm a big NFL Draft fan, and the last few years when ever a defensive lineman is picked and discussed, the "technique" talk starts flowing.  I briefly touched on the various technique numbers and gaps across the defensive line in; Tampa 2, Cover 2; What do the Bears do?, but this post will be dedicated to the defensive line.  Stuff like those D-Line gaps you hear color analysts talk so much about, and the technique numbers and what's expected from the types of linemen you have at each spot across the line.

In the diagram above the circles represent the offensive line and tight end.  There could also be a double tight end formation, the tight end on the left side, or no tight ends at all, any way the offense lines up the technique numbers don't change, nor do the lettered gaps between the offensive line (circles).

Obviously I'm not allowed access into the defensive line rooms or the team practices, and the Bears aren't posting their defensive play book on the web anywhere, so I'll do my best at breaking down this stuff, and of course it'll have a Chicago slant, this is Windy City Gridiron after all...

The last few years for us Bears fans the term "3 technique defensive tackle" is something we've grown accustomed to hearing.  The 3 technique DT (sometimes called the under tackle) lines up on the offensive guards outside shoulder and is responsible for shooting the B gap.  Chicago usually has their left DT (lining up against the offensive right guard), as the 3 technique.  The Bears right DT then lines up at the 1 technique on the centers shoulder and shoots the A gap.  You may hear this DT refered to as the Nose Tackle on occasion.  Sometimes the two DT's will flip side of the ball depending on offensive strength.  Ideally the nose tackle will be a little bigger and stronger in hopes of requiring a double team from the center and left guard, but he still has to be quick enough to shoot his gap.  Obviously you'd like both your DT's to draw a double team, which will give your LB's more room to operate and leave both defensive ends in a one on one situation, but drawing double teams is more a byproduct of having really good DT's and not a staple of the what the Bears do defensively, because...

The Bears are a one gap defense; get through your assigned gap and don't let anyone run by you, either make the tackle or redirect the back to an awaiting teammate.  In a two gap defensive scheme you ask your defensive lineman to read and react at the line, while occupying the gaps to either side of them.  Sometimes these two gap players line up head up vs. the offensive guard and are told to hold their ground.  For example in a 3-4 defense the Nose Guard (usually a 0 Technique) has to be big and stout enough to shut down the inside running game of both A gaps.

The defensive ends in the Bears 4-3, depending on the down, distance, and offensive formation, could line up anywhere from a 5 technique to a 9 technique.  The 7,8, and 9 techniques aren't on the diagram above because many teams have different definitions for these numbers.  The 9 technique in any system will be the furthest outside a DE will ever line up, usually a couple yards past the last offensive player on the line of scrimmage.  On most plays the Bears ask their defensive ends to penetrate through the C gaps.  The 4-3 defensive ends are also asked to play contain, which simply means not letting the ball carrier get outside, so in effect the DE is also protecting the D gaps just outside the TE (see pic below for more gaps).  Your outside linebackers better be fast enough to cut off anything outside (E gaps), a sideline to sideline middle linebacker will help too. 

The prevalence of the Zone Blocking scheme in many offenses today has the running back reading the block on the defensive end.  If the player assigned to block the DE turns him in (sealing him) the back will take it outside.  If the DE can't be sealed the back will turn it up and get what he can.  Lining up your defensive ends in either a 7, 8, or 9 technique makes it that much harder to run to the outside.  If the DE can't make the play he needs to either run with the back (stretching the play to the sideline), or force him back towards the middle where most of his teammates reside.  You'll hear some analysts call this holding the edge.  One last responsibility of the DE in the 4-3 defense, regardless of where he lines up, is to stay home on running plays away from him.  Often times an offense will try and induce a defense one way, then counter to a back, handoff to a wide-out, or run a reverse away from a defense's pursuit.  The third aspect of the zone blocking scheme is a runner cutting back against the pursuit, a good DE will stay home and thwart any runs back in his direction.

One last point about gaps, if the Bears have their base 7 defenders in the box, each player will have a gap assignment.  The 6 gaps, A's, B's, and C's, are the responsibility of the 4 down lineman and the 3 linebackers.  Seven players to control 6 gaps, which usually leaves one linebacker a little more freedom to chase down the ball (as long as the other 6 are controling their assignments that is), in the Bears defense it's usually the weakside linebacker that has this responsibility.  When the Bears bring their strong safety up as an 8th defender, often times it's because one or two players can't control their gap.  Last year the Bears played a lot with 8 in the box, which was a big reason their pass defense was so suspect.

As long as Lovie Smith is the Head Coach of the Bears, you'll never see GM Jerry Angelo draft big hulking defensive lineman.  With the one gap defense Lovie Smith uses, it's all about their speed and quickness.  One reason Bears three technique DT Tommie Harris is such an effective player when he's healthy, is he has unbelievable quickness off the ball and at a lean 6'3" 295 he has the ability to overpower many offensive lineman.

I could honestly go on and on about this stuff (it's the coach in me), and I'm sure I rambled on a few times, but for more general info on this stuff here are a few links I came across that I enjoyed.

Football 101

Fox Sports

Athlon Sports

and even Wikipedia has some good info.