You will never have more than 3 games in a week. Usually this is 2 league games and one scrimmage. A game is 48 minutes long, unless there are overtimes. So during a training week there are 144 (48 x 3) minutes of playing time available for training each week.
Training is based on minutes played at a position, not on games played. A player may play more than one position in a game, so only some of his time might be credited to training. The Weekly Stats menu will help you track minutes played each week, once you have played games.
Theoretically, you could split the 144 minutes among 3 players. But assuming you want to win your league games, your best player at a position will play more than 48 minutes a week (perhaps 60 to 80 minutes in the two league games). He won't play in scrimmage. His backup then might have 16 to 36 minutes in the two league games. So he would need some more time from the scrimmage. And you would be able to get some minutes for a 3rd player, but probably not 48 minutes. You might want to develop a younger player who can benefit from some of the extra training.
There is no benefit for training purposes of playing more than 48 minutes per week, so even if the 3rd player doesn't get the full training, the other two players would not get further benefit.
You could also train a player "out of position". Most (all) of the skills used by a C are useful to a PF. So you could use one of your starting or backup PF as a 3rd C trainee.
Playing minutes are also important for Game Shape and Stamina. A player who plays between 48 minutes and 90 minutes in a week, regardless of position, will improve or maintain his game shape. If he plays too much or too little, his game shape will deteriorate. The further outside the range, the greater the loss.
A player who plays less than 48 minutes in a week, regardless of position, will have his stamina deteriorate. Again, just missing 48 won't hurt too much. Playing 0 will hurt it. Stamina can only be improved by training stamina. If a player plays more than 48 minutes, it will simply stay the same.
Because of the Game Shape and Stamina (and salary) it probably doesn't make sense to have more than 12 to 14 players on your roster. If you have more, their game shape and stamina will suffer.