Since this requires the CPU to put in a pinch hitter it's a bit uncontrollable but there are a few things you can do to improve your chances.
First, turn off the designated hitter (DH) so that pitchers will bat, as once their pitcher will come up to bat that will be the most likely situation for a pinch hitter (PH). This will be especially more likely once the starting pitcher (SP) has been taken out of the game and the CPU is now using relief pitchers (RP/CP) for only 1-2 innings at a time.
Second, once the CPU does put in a pinch hitter, make sure you are using a good, fresh pitcher on the mound. So if your SP has been on the mound for 4+ innings, or a RP for 2+ innings, change to a fresh RP to give you better odds of striking out the batter. (I know it's more ideal to have SP go 5-6 innings but this is just about giving you maximum odds for a 3-pitch strikeout.)
Third, just follow usual advice for strong pitching, such as make sure you mix up the pitches, and in this case keep pitches within the zone as you don't want to bet on fooling them, and nail the velocity/aiming with the reticle. If you can nail 3 strong pitches in the corners with 99 and great/perfect aiming, you should do fine.