A3P (Attack in 3 Pitches) was created by Missou as a way to track their pitchers ability to "get ahead in the count."
Various schools use it in a different way than Missou originally created it for their program but the essence is the same. We record each pitchers A3P per batter. It gives a metric that if the majority of the batters are behind in the count after 3 pitches, there is a higher chance for an out. Balls in play are considered "strikes" no matter where they were in the strike zone. Of course the metric itself is not a "tale tell sign" of complete success, but in developing pitchers we want them to work from an "ahead" position verses a "behind" position.
It is a way to judge if you are throwing strikes basically and getting to a 1-2 or better count against the batter.