Players will be trained when they play minutes at the position that you train. The amount of training they receive increases as they play more minutes with 48 minutes giving 100% training (so if they make more minutes, they still receive the same amount of training). Because you play 3 games in a week, this means you have 144 minutes of training per position you train (if you train 1 position, the training has more effect). If you manage perfectly you could give 3 players exactly 48 minutes playing time in 1 position, so you can at most give 3 players maximum training. Because it's very hard to manage that, usually managers train 2 players.