I'm training Guards throughout my entire bb-career (at least until I figured out how to train properly). And I must say, these trainees aren't that bad. The two all-stars will cap at around 22 or so, the superstar maybe 1 or 2 seasons later.
By that time you can make decent money with them and keep one on your roster.
Regarding training - since you have exactly 3 trainees, I'd recommend one-position-training only (OD, PA, JR). Once you have finished these skills you switch to two-position-training and get 2 or 3 more trainees, to not waste any training-minutes.
To get an idea of how they might look like after a couple seasons, check out the most expensive players on the TL with the same potential. Those players would have about the maximum skill-set.