The Three Kingdoms actually had pretty different approaches to warfare, despite all being mainly Han Chinese.
Wei were the powerhouses, possessing the most "industrial" base and Imperial forces; they'll be the old army of China, so heavy shock cavalry and pikes, maybe bows/crossbows. Maybe chariots as well, but that'd be a stretch. They also hired huge numbers of the tribes who would one day become the mongols, so expect light cavalry and horse archers to be among them. They'd probably have the most 'normal' army, relying on heavy armor and formation-fighting to bring victory.
Shu was primarily a guerrilla-ish force, an "army with a state" rather than a state with an army. They were mobile, adaptive, and generally fairly small in number; I suspect they'll rely heavily on ranged units and ambushes, medium infantry and light cavalry. Later on they'll likely incorporate the Nanman, but I'm not entirely sure how they'd fight. Repeating crossbows and weird things like that, too - after all, Shu invented them. Maybe they'd have a lot of weird elite units that you can only get a few of. Gimmicky shit combined with versatile units. Very much doubt they'll be a horde, though.
Wu were mostly focused on their navy, as their primary defense was the rivers they controlled. They'll probably be heavy on shock infantry and archers and lots of boats. Ideally we'd have riverboat combat so that their river fleets could properly fight 'land battles', but we'll see. They were involved heavily with the Shanyue people, and were somewhat culturally distinct; they changed a lot of southeast China, turning it from a 'barbarian' region to one far more civilized in the eyes of the country as a whole. Plenty of room for doing cool stuff with Wu.
It's one of the hinging points of the novel, actually; Wei rules through "heaven", as they control the Emperor and are the legitimate successors legally. Shu rules through "man", because they're a populist leader and folk hero who claim to be the legitimate successors morally. Wu rules through "earth", because they have terrain and wealth on their side, and were different culturally and ethnically from Wei enough that they eventually tried to break from China entirely. Which I hope is included in their campaign; it would be interesting to point out that not everywhere in China agreed with China being a persistent thing yet.
Additional factions could include the Xiongnu proto-mongols, the Jurchens, Tibet (Sadly too early for the awesome Tibetan Empire), the Thai and Nanman... Basically all the tribal / smaller kingdoms who bordered China at the time, though their main interaction was being hired as mercenaries. It's too early for Wokou pirates, I believe, and Korea was going through its own, entirely unrelated Three Kingdoms period. It almost seems like the map will include Iran / Persia and Central Asia in general, which would be interesting, but I'm not sure what reason they'd have for being included. The Kushan would be on their last legs as the Sassanids rise.
There are also the other warlords, of whom there were many - Yuan Shao, Dong Zhou, Lu Bu, and myriad others all seized power at one time or another, and depending when the game starts could be viable factions. Jin seems less likely, except as some kind of late-game crisis or another.