These three mons are some of the best mons for a pivoting team to ask for. Volturn has the potential to bring in a ton of abusers like Abomasnow, Drampa, Indeedee-F, and Perrserker to name a few, and pivots are essential as these wallbreakers may lack switchins otherwise. To help, Watervally, Clef, and Rotom-Frost offer pivoting in three different forms.
Watervally is a beast in its own right with that high powered Multi Attack, but with U-Turn and Swords Dance you can also dent Grass-types. This helps as any check to SD can be in some serious trouble when bringing in a wallbreaker.
Clefairy with WishPort is somewhat passive but really helps getting the team back to full. Watervally may take a lot of damage due to its natural bulk and switching into hazards, and Wish can really help.
Rotom-Frost finishes the core off with a threatening boltbeam coverage and pivoting. It also helps that it removes hazards while crippling setters like Piloswine, Ferroseed, and Garbodor, if not KOing other setters like Sandaconda with Blizzard.
These three can support an offensive VoltTurn team really well while still providing good defensive utility thanks to their bulk, typings, immunities, and supportive movepools. Some vulnerabilities like lacking a Grass- or Poison-type resist can easily be covered by the wallbreakers you wanna pivot into anyways like Drampa, Perrserker, and Golurk.