I would look to pick up two or three more trainees, then train Rebounding and One on One for forwards. One on One will train Driving, Handling, Jump Shot and Inside Shot. I really hate atrocious driving on any player-it's useful for bigs to be able to open up their own shot in the lane. Jump Shot is needed for centers as well, even in Look Inside offences they still might need to take a short-mid range jumper. Since One on One and Rebounding are both 2 position trainings, you can get full training for up to 6 trainees-this is why I reccomend looking for 2 or 3 more.
Once you've finished with One on One and Rebounding you'll have to decide how to proceed-I'd Reccomend Inside Defense first, but if you want to 2 position train that you'll be sacrificng some training speed-informal experiments show it to be about 80% for 2 position compared to 1 position. So you'll have to decide if you want to sacrifice training speed, or sell off some of the trainees to go back down to 3 total.