My thought is to train all 3 to at least 13 or 14 in OD first. Then I could rotate JR/PA/JS afterwards. Would that be smart? I am using a Superior Trainer and single position training currently.
I'm gonna agree with some of the above and say it's kinda silly to just train the OD up to the tremendous-wondrous range while none of them can really shoot. Not to mention that the 18yo is a full 3 levels behind in OD, so even with slow-down due to age, they likely won't all be 13-14 at the same time. Either both 20yos will be higher or the 18yo will be lower.
This vague way of referring to them is bugging me, so I'm forgive me if I name them thusly:
Player A
Age: 20
Height: 6'0" / 183 cm
Potential: allstar
Player B
Age: 20
Height: 6'5" / 196 cm
Potential: allstar
Player C
Age: 18
Height: 6'2" / 188 cm
Potential: perennial allstar
It looks like you're currently training JS, so you've started to address the main weakness of all three, which is their perimeter shooting. B and C have good JS but issues with JR, and A has decent range but low JS. Other than that, they're all nice trainees.
But here's the thing: B is probably never going to be a great guard or SF. That low range is holding him back. Unless you want to try to make him into a nice PF, you should probably sell him soon. The reason being that the other two will grow into fine guards while B lags behind, unable to shoot well from behind the arc. B is also the only one tall enough to bother training inside skills on, and he has a great set of starting skills for inside training.
If you're not prepared to sell B right away, focus your training on C (he's going to be the best in the long run anyway) and let A and B pick up whatever pops happen along the way.
Just my two cents.