If I were to sell all but one to go to 3 position training, which one should I keep as the 3rd trainee?
NEVER 3 position train. As for what would be sufficient at the D2 level, it depends on if you want to sell for profit or play and train. A last resort should be PA potential, but superstar would probably be my cutoff if I were in D2. You are only in D4, and I don't think you should worry about it until the time comes.