I think 1 on 1 is always better than training HA except for more "obscure" cases - for example, you don't want your player to train JS so he doesn't cap.
If you have only three trainees and want to train 1 on 1, I suggest this: Buy three trainees for cheap, include them in 1 on 1 training and when you are done with 1 on 1, sell the three trainees for profit.