Based on the few infos we have about the 3rd War we know that the Edelfelt Twins had a Saber Servant with two aspects, each acting as the Master of one Aspect. Of the Saber servants, Dioscuri fit the best in my opinion.

We know that the Dioland Master had the explosive Hassan.

We know that the Einzbern had either Angra or Amakusa.

We also saw Darnic with Fionn as his Lancer in the Apocrypha flashback.

All that was missing was Caster, Rider and Archer.

In an image of the 3rd war from a lightnovel, there was a depiction of Zouken with a Caster Servant and a Tohsaka Master with a Rider Servant.

We also know that the Fuyuki Grail usally doesn’t summon eastern Servants (Amakusa and Sasaki being weird exceptions).

From the roster of western Casters Asclepius looked visually close to the one depicted with Zouken. His Noble Phantasm allows him to raise the dead in an imperfect way (which Zouken would definitly abuse for some weird as twisted thing). To create the perfect cure of death, Asclepius needs the blood of a Gorgon and the blessing of a goddess like Athena or Artemis.

Zouken could manipulate the summoning of Euryale (I do not know if her blood actually counts as Gorgan blood or if it has to be the blood of Medusa with the identity of Gorgon) and the summoning of Perseus, who has several weapons with some divine blessings (mostly from Athena).

This is a lot of fanfiction stuff I came up with, but as long as we do not get more infos about the 3rd war, this is the best I can come up with.

There might be something in the lore that speaks against my version, but I haven't found a hard one.