How can i train exactly those three? i only get the option to either train 2 of those together or then the whole team. (wingmen, guards etc..)
Yeah, you're right. You have 2 positions that receive training in JS. So I would make sure I had 6 trainees (SG and PG only). Training PF at the same time is quite complicated as the you train both in JS without playing some totally out of position. And that will most likely hurt your performance.
So I would definately go with 6 SG/PG trainees, which means you would have to buy a few more. This will make you the best profit, and will also give you better results.
Shuffling between outside and inside training is not a good idea - at least not in the beginning. I tried it for a short time, but just to give my SF and PF a bump before going back to outside skill. But this requires a couple of very good player in both positions, as you are sacrificing for some of your normal outside players. I think this is only adviseable if your main trainees are your SF's.