4 day simple workout

Could you tell me the theory behind theĀ  3sets of 3 then 3 sets of 6 then 3 sets of 12

The 3 sets of 3 seems very simple and light exercises.

Its known as Liner periodization when you start wight hose sets and reps and progressively increase the reps over time. The 3 sets of 3 reps generally is pushing for heavier weight to be used. In most instances when I write I will use this set and rep scheme for exercises that are multi joint, and or strength or power moves that have you exerting a tremendous amount of energy and correct technique. Power Cleans, Squats, Deadlifts, and even bench Press are the most common movements where I use this scheme.