Although a combination of these can be useful, especially if your 3 big trainees need JS, HD, DV and REB training since you have to 2 pos these positions.
If you have 2 or 3 players that just need those trained or can benefit from those then 2 pos can be useful. But for IS and ID, you want to 1 pos that training.
You really need to first decide what you need trained. If your bigs are too sloopy (low HN) and drop a lot of rebounds, you might want to do 1o1s then you'd need more guys, if not and they just need a lot of IS and ID then go with the big 3. Although eventually you'll need to train REB and getting as many guys as you can in on that training is fiscally advantageous.