Mulumbu at the moment would draw the most out of them three IMO. He is young, consistent, doesn't really get injuries and is a rare commodity - a "defensive" midfielder who also scores a few goals! He is brilliant and I can see some big teams coming in for him, if not this summer than the next one.
GD hasn't quite had the season we all hoped but I still think he will be class. Next season will be a huge one for him, please God we will still be in the PL. I think Roy definitely likes him and he will be a 1st choice midfielder next year, and hopefully he can live up to expectations. If he does that, he's the kind of player who would be worth £8-£10m in today's market.