Definitely not player C. I think, contrary to everyone else, I'd say player A. I'm the kind of person who wouldn't be up for working with that pitiful OD and awful Handling, and A is a lot better starting with those skills, in addition to having better inside skills.
If he's just a tertiary trainee and isn't your main focus, you're not going to strive to make him the best player he can be anyway. So the third guy is definitely out. Star potential is still high enough you could realistically work the 19yo to cap, and while you could cap the 18yo as well, I think it'd be a bit rougher starting out, especially seeing as his defensive skills are so low, which in my experience would lead to foul-outs and make training that much harder.