depends a bit, i think in generell JS would be better cause this should have priority for trainee 2 and 3, while JR have priority with trainee 1.
But you have three trainees, so maybe you focus first on the staff you can train on 1 position, and when your minutes are bad switch to two positions trainings. And maybe get 2-3 trainable player in 2-3 season and then make a lot of JS and 1 and 1.(these guys should have OD and Passing already)