Okay.
Metagross and Sylveon do go great together. There is one change you might want to try on Metagross, and that's Psychic over Psyshock. This is because of higher power and you can already hit special defenders with Bullet Punch. Maybe, however, you could try Shadow Ball over Bullet Punch (and try 4 SpDef instead) and then run Psyshock.
Sylveon seems great though.
As for the other two members....
Well, Metagross would be weak to special fire types without the Assault Vest. I just think that's a great doubles combo. Not sure where to start, but maybe a more supportive Pokemon would be nice.
Tapu Fini @ Aguav Berry (or something you might think of).
EVs: 252 HP/124 Def/124 SpDef/8 SpAtt
Calm
Ability: Misty Surge
Moonblast
Heal Pulse
Scald
Reflect
This is a set I've been running in the Ultra Moon battle tree. Reflect helps Sylveon's fairly low defence. Heal Pulse is good for Metagross, so you can stay in battle for more time. I know two fairies might not be great, but Tapu Fini is a great Pokemon.
Finally, we have an supportive physical attacker.
Incineroar @ Figy Berry
EVs: 252 HP/4 SpDef/252 Attack.
Careful or Admant (you have Careful right?)
Ability: Intimidate
Knock Off
Fake Out
Flare Blitz
U Turn
Incineroar is a great Pokemon. If you have a defensive set, that can work well too.
Hope I helped!