If you're sure that you can have your young player stay at least 2 seasons with you, then use him. And try checking the average match rating and statistiques for who performs better. Third option is to place the three of them on bench and buy a better one =D
I have many players from season 1 playing till now (season 5) and their stats are really impressive. My fast trainer ST was 19 at season 1 with a 15 of quality so I trained him till now and he's now quality 33 League top scorer, rated and assiter. He's one of the weakest in the team at the moment but his experience with the club has made him a pro player.
So it's up to you for who to make play. I'd go for a new dr age 18 if possible and I'll leave him for few seasons.