I know you don't believe in skills, Nik, but I do =]

MR and AMR are both attacking positions, so they are more or less similar, but DR is completely different. These three positions have only three skills in common: Crossing, Fitness, and Speed. Fitness and Speed aren't the problem (never was), but Crossing is. For DR the best drill for training Crossing is Defending crosses, for MR it's, arguably, Set-piece delivery, and for AMR -- Fast counter-attacks.

It is very much possible to have DR/MR/AMR, but he will be less effective. I try not to mix defenders and attackers.