yer this is all true, i think focusing on 2 trainee's is as much of a psychological thing as it is practical.
Having only 2 trainee's makes it much more clearer about what your goals are for each individual player, and provide less distraction for getting them minutes or training something that goes against your training plan.
What I prefer to do, is have 2 younger trainee's that I can maximize the 48 mins against, and then I have an older trainee whom gets the supplimental minutes.
So at the moment i have Nash and Bellard and im smashing ID and SB. but ill throw in 1n1 for forwards every now again. My third trainee is Phil who is 25, but has MVP potential (and he was a SF prospect) - so he gets the minutes - but only sometimes because im not willing to sacrifice my league position just so that he gets minutes at C.
so yer, sometimes i have 3 trainee's, sometimes not. Bellard is not good enough at the moment to play in league games, and Nash only become good enough halfway through the season, so its made it difficult, especially because I was also in the cup
So i think having 3 trainee's is possible, but it depends on the state of your trainee's, and your league expectations. But for, Div II with 3 x 18 or 19 year olds who are brand new trainee's - your league and cup success is extremely limited.