This is a huge difference when moving to 3B. You have to be so much better going back on balls compared to SS. Everything gets on you quicker. Most grounders have less bounces - so they get on your quicker and stay hotter longer.
3B grounders have way more spin on them than SS grounders. Top spin makes the hops play different, and most of the time you only get a couple at 3B to create your hop.
You need the ability to go back on balls to get the hop you need to get. There’s way more ways to get stuck in between hops at 3B. The best can drop step to create time, create angles, and create the hop they need.
It’s also huge for getting your eyes to the side of the ball - which is key to fielding balls clean and getting good reads when EV are as high as they are at his level.
He’s training the right things.
Ah yes, classic baseball fundamentals of running away from the ball while trying to field it