my three trainees need to play 48 minutes to get training, but it's impossible because always one trainee play less than 48 min
Do you mean you train only one position (e.g., PG)? It's pretty much impossible to give three guys full training in a single position. What you can do is set each guy as the starter, back-up, and reserve for a single game and use the strictly follow depth chart order. Your players will get tired and you may have nasty surprises in case of injuries or foul trouble, but that's pretty much what you need to do if you want 48 minutes for three guys in a single position. Or make the BBB. :-)