I've been wondering lately whether it was worth playing only one of my 3 trainees seperately in each of the 3 games so to maximise training. I mean if you have 3 players in your training position of a similar standard who recieve a similar rating i can't see why it wouldn't work out. Atleast you could be sure they were getting maximum training. I wouldn't do this for 2 players per position however.
AT OP. You want to make sure primarily that in the positions you want to train you have the youngest people you can find, this way you are maximising the training speed opportunity and are going to gain levels of training much quicker.