Why would you expect to receive more overall training for more positions?
A trainer only has 24 hours in one day to train players. If you decide one player should receive all 24 hours of course he would benefit more and quicker than if you decided to train 3 players for 8 hours each.
That is where the strategy lies. Do you want a single force or a well rounded team?
In real basketball, you need 8 or 10 players for most of the drills. For example, how do you expect to train pressure with only 3 players?
With less training players, the trainer can be more focused in those players, but you need around 10 players for a good training session. (except for some technical issues)