Ah yes, the dark three headed dragon, Hydreigon...
For starters with Hydreigon, you would need to focus on Special Attacks more than Physical attacks, as Hydreigon can reach 383 in Special Attack with a Solid Nature such as Modest or Quiet And 31Iv’s with 252 Ev’s
But you’ve come for the MoveSet, haven’t you? Let’s go!
For Hydreigon I believe a solid Dragon move is in order for starters, so I would recommend either Draco Meteor or Dragon Pulse.
If you wish to smash the opposite Pokémon by all means use Draco Meteor, If you wish to keep your special Attack the same, use Dragon Pulse
For the second move, I’d recommend Dark Pulse as a solid Dark Special Attack which can hit HARD.
For a third move I would recommend Earth Power as it can lower Special Defense, as well as pulverise some of the fire types, rock types and Steel Types that you would often fight in competitive play or online battles.
For a final move I would strongly recommend going for either Fire Blast to burn away Ice types and Steel Types that refuse to be pulverised by Earth Power...that or Flash Cannon that can take care of pesky Fairy Types And Rock Types That dominate the playing field.
So there we have it, a move set worthy for a King Ghidorah-Esque Pokemon
Dragon Pulse/Draco Meteor
Flash Cannon/Fire Blast