In Basic terms, I treat the Gun Quick Passing game drop as a 1-step drop. As, there is no need to get depth while throwing from the gun, the one step primarily allows the QB to get is feet properly set, buy just enough time to get his grip, and to make his read in order to get the ball out quickly and on time.
In my quick passing game, the QB must decide which side he will throw to BEFORE the ball is snapped based on pre-snap keys. You generally do not have enough time to check both sides of the field in the quick passing game. Every now and then you can get away with it, but you do not want to get in the habbit of checking both sides. It just takes too long and creates sacks, fumbles, interceptions, etc.
From under center, our 3 step is a little different depending íf the QB will throw right or left:
to the right (for a right hander):
1. 1st step is always for depth. You want to get as deep as possible, as fast as possible.
2. 2nd step is a shorter control/brake step, not worring about depth here.
3. 3rd step is to get the weight transferred from back to forward, and more exactly, directly to the target. The QB needs to get s is 'energy' in one smooth motion from moving bacwards to moving forward. I often refer to this as 'getting aroung the corner'
to the left (for a right hander):
1. 1st step is exact same as above.
2. 2nd step is simular, but now as he crosses over he sets his foot a little back to the left. This initializes the energy transfer.
3. 3rd step is a short step sticking the right foot behind him to the left. Very important here the the step is a small one pretty much remaining within the frame of his body. The back leg should form a 'Z' shape and absorb the his 'energy' or weight, allowing for a smooth transfer.
It is VERY important regardless of throwing right or left that steps 2 & 3 are smaller controlled steps. If the QB uses larger strides, especially with step 3, he will have his 'energies' going in different directions and be off ballance.
For longer developing 3 WR route combinations from the gun, we may use a very short, quick 3 step to allow the QB to make his proper read and for the play to develope.