Well, if you're going to get a guy 48+ minutes in a single game, as Dodor pointed out, you must dress only 9 players, and run the risk of foul outs and injuries. There's no other way to do it
The problem with the way you set it up it that your trainee could be substituted into a different position in his reserve role, which kinda defeats the purpose, no?
Single position training and trying to get 3 players their 48+ minutes every week involves an inherent trade off between the training and your team's performance in league games. You could instead only focus on two main trainees, and then it would be pretty easy to get 48+ minutes at the training position every week.