yep this is the way to have 3 trainee's.
Mickyster aluded to maybe only having the 3rd trainee as a 'lesser' trainee, because what may happen in G1 or G2, is that your trainee does not get his allocated minutes,
So say Trainee 2 only gets 40 minutes (for whatever reason which i won't go into here. ) in G2, then you could set him as reserve and backup in G3 to get those extra 8 minutes.
However the impact is that then your trainee 3, only gets 40 mins.
Also the problem with having 3 trainee's, is that you end up with 1 position in every single game, which is, for a few seasons, having (possibly) a lesser player in that position, week in week out. - That is to say, you put a brand new trainee at PG, its going to be 1-2 seasons before he is servicable as a PG fo ryour team.
So be aware that training brand new trainee's can be determinantal to your team's success. So this is all part of a being a sucessful manager, and managing/balancing this.