Here is what I would do if I were in your situation:
1. I would sell your 2nd trainee because he is a guard and he is 20 so he doesn't fit with the bigs that your are expecting to train, you'll make good money on him for impact players, another trainee, or further development in arena.
2. Buy another big man trainee with some of the money you got from selling your guard trainee with similar primaries as the 2 your going to train.
3. Focus on boosting primaries the whole season, try to train stamina on ALL-STAR week and another week that you feel comfortable with if you think you want to have your 3rd trainee be a future starter in a few seasons.
TIP: Since your in a low league, take that time to develop arena!