When you say "leader" do you mean core players to build around your team? If you like training, try finding some balanced 18 year old players (all-star level at least) that do not have many atrocious stats (skills) they will train much faster. They will surpass those 3 by age 20 or even at late 19. One position train as much as you can with at least an advanced trainer. Three trainees is very difficult to train properly, try considering just having two and you can keep the 21 year old as a back up trainee.
Each trainee needs 48+ minutes not just 48 in any given week to maximize training. At 26, he will just be a complimentary back up and training will be very slow. Try using the right arrow in the forum section, there are a lot of training threads/discussions that you might find useful. Hope this helps.