A defender is lined up in a three technique if he is one the line of scrimmage across from a guard’s outside shoulder.
Any defender can theoretically be in a 3 technique, but it is almost exclusively a defensive lineman, specifically a defensive tackle. Some defenses have players who are designated 3 techniques in the same way an offense might designate a left guard. Other defenses only have defensive tackles who may line up in a 3 technique. The more sophisticated the football and the more specialize the players, i.e., the NFL, the more likely you are to have the 3 technique be a specific player as well as his alignment.
All positions along the defensive line can be understood through a numbering system. The defenders are either directly across from an offensive lineman, called head up, or they are on a shoulder, called a shade. Each position has a number, as follows:
Even numbers indicate head up positions, and odd numbers indicate shade positions, most of the time. The inside of the guard and inside of the tackle are both denoted with an ‘i’. Why is that? It just evolved that way.
Gridiron Glossary is a resource for football terms that are often used by commentators, coaches and players but rarely defined. If there is a term you have questions about or a definition you don’t agree with, let me know at firstname.lastname@example.org.