The phrase of the day is 3 by 1. It’s one of those words that can have various meanings from offense to defense, or from coaching staff to coaching staff.
The basic meanings are shown below.
Offensively, 3 by 1 is usually defined as a 4-wide look with 3 men on the left, and 1 on the right. 2 by 2 would be 2 on each side. 1 by 3(which is shown in a bunch formation in the defensive section) dictates the opposite.
Defensively, it’s more of a slang term form a nickle defense where the 3 corners (LCB+RCB+NB) are on the same level and likely playing a zone with the FS in a Cover 1. Again, each coach builds their own vocabulary and it has been used for a 31 front 3 DL by 1 LB with 7 DB’s on the field.
One of the beautiful, yet confusing thing about football is that words, terms and phrases can have multiple meanings.