Yes I think to have 3 trainnes is good. If we could have 3 trainees with the same profil in skills, it's better. 6 players is too many, and is not effiscient.
With 3 players we coulf build a team around him. And if each manager have the same idea (have 3 trainees), in U21 we could build a team and test tactics and orientate the profil of attacks and defense.