Sorry i didn´t explain myself very well.
Game 1: Starting A, backup A, reserve A
Game 2: Starting B, backup B, reserve B
Game 3: Starting C, backup C, reserve C
If no player is fouled out or injured, they´ll play 48 minutes each one, at the position you train, and they´ll receive full training.
Additionally you can only list a maximum of 9 players in the "Selected Players" at the top of the set lineup page. If there is a 10th player available - even if he's not placed in the depth chart - he'll come in to replace your trainee in blowout situations (20+ leads in the 4th quarter).
Example for PG training
PG: 1 player listed as starter, backup, and reserve
SG: 1 player listed as starter, 2nd player listed as backup/reserve
SF: 1 player listed as starter, 2nd player listed as backup/reserve
PF: 1 player listed as starter, 2nd player listed as backup/reserve
C: 1 player listed as starter, 2nd player listed as backup/reserve
Total of 9 players eligible to play in the game. Strictly follow depth chart, and for fouls choose "Let them play".
This should give you 48 minutes for your trainee so long as they don't get injured or foul out, but can lead to some strange placements if that happens. A C playing at PG as an example.