I mean, based on where volume 100 ends, it's very likely the arc is going to continue through volume 104 / 105. There's plenty of time for covers with characters like Jack, King, and Queen, even if it's not a dedicated 'villains' cover.
Love what Oda's doing here and that he continues to surprise.
I don't think this being a three part cover means we're any closer to the end of the arc than it seems at this point in the manga itself. Oda has no exact pattern as this three part cover proves. The first time he did a villain / hero pair of covers, there was only one volume remaining.
Volume 21 - Villains
Volume 22 - Heroes
Volume 23 - End of Alabasta
The next three times, there were two volumes left
Volume 42 - Villains
Volume 43 - Heroes
Volume 44 - Last volume of the fighting
Volume 45 - Falling action / exposition / end of CP9 saga
Volume 77 - Villains
Volume 78 - Heroes
Volume 79 - Last volume of fighting; Luffy vs. Doffy
Volume 80 - Falling action / end of Dressrosa
Volume 87 - Villains
Volume 88 - Heroes
Volume 89 - Last Volume of fighting; Luffy vs. Katakuri
Volume 90 - End of WCI is in the beginning of the volume; contains the Reverie
So the last example, technically Reverie isn't part of the WCI arc; So you could say it's sort-of an in-between case;
Baroque Works - 1 full volume remaining after Heroes / Villains
CP9 - 2 Volumes (1 fighting, 1 falling action) after Heroes / Villains
Dressrosa - 2 volumes (1 fighting, 1 falling action) after Heroes / Villains
WCI - 1.5 volumes (1 fighting, 0.5 falling action / 0.5 Reverie) after Heroes / Villains
Here we have it spread over three volume covers
Volume 99 - Big Mom, Jimbei, Usopp, Chopper, Robin
Volume 100 - Luffy, Zoro, Law, Kid, Killer (?)
Volume 101 - Kaido, Nami, Sanji, Franky, Brook (?)
I'd say Oda likely chose to do the 3 part cover now because of the 100 volume milestone, moreso than as a marker for how close we are toward the end of the arc. I can easily see 3, maybe 4 volumes remaining in Wano (with 1 being primarily falling action) after the three part, particularly if there are 5 acts (but even if it's only 4).