I don't know how Fred runs this, as I've seen a couple of variations of it, but I don't care if he's just standing outside the arc, able to spin the ball, relax and shoot it like a free throw, that is a crazy percentage. If he's running that drill with a mix of sprints and shooting from 5 spots like a lot of coaches do, it's really quite spectacular.
I was watching a coaching buddy run his kids through a 3 pt drill. It was pretty simple, but the emphasis was to adjust your shot as you fatigue. Run the lines twice, shoot 3 pt jumpers (no toe raisers or sets) from 5 spots on the arc, until you make 10. Then back running lines. The number to make is 100. As you can guess, some guys were getting gassed and really struggling towards the end. Form starts to go, and shots turn into heaves. It’s good to learn how to adapt your game when you’re tired, it’s also good to know who you can count on late in a game.