This method is the best if you want to maximize your training minutes in a week, but to have succes in the same time, you need your three trainees (or two of them if you're no longer playing the cup) to be at a certain minimum level in order to play 48 minutes and not making you loose your game...
Personally I can't, I think that it's more efficient to have a main trainee that will be your starter in your league games and a second one being is back-up, and of course, turn things around for the cup/scrimmage.