Practice with light loads. That way they won’t flinch when shooting heavy loads at turkeys! They won’t feel a thing when actually hunting and much more likely to hit the turkey! Good luck!
I remember when my youngest boy was 10 years old and he practiced with a 1 1/8 oz 7 1/2 shot (trap round)... I kept the same shell in for hunting. Called in a tom to 10 yds and he smoked it with a trap load. Don't think you need to get the biggest baddest shell out there. The next year I had him shoot a pheasant load of #5 shot and worked just as well. Good Luck.
Be careful with those 3in. They kick like a mule though those little guns!