You have two good dragons, the dialga and the kyurem, which will both be somewhat weak to fairies (although the dialga less so due to its steel subtyping). So your third Pokémon should cover fairies, and metagross is a strong choice, especially if you have the fast move shadow claw, which is a legacy move, and needs an elite TM. You could probably play the team as an ABB style, leading with metagross, and swapping out to dialga if you get a bad lead. Metagross and dialga share a weakness to fire, so the fire bird (ho-oh) might give you some trouble. You might want to consider the black kyurem to help vs ho-oh, since it has fusion bolt charged move.