Some time in the last year I read a novel in which the protagonist tried to read this, and bounced off it. (I'm having trouble identifying which novel – a search of my electronic copies of likely suspects The Fat Years and The Three-Body Problem doesn't pull it up, so it must have been something else, possibly a Clarke submission.) I winced a little when I came across that reference – the group of friends with whom I read War and Peace in 2012 and Anna Karenina more recently had also tried The Romance of the Three Kingdoms as a collective reading project in 2013, and we ran into the sand pretty quickly. For those who don't know China or Chinese history, the names of people and places are pretty baffling; and the plot seemed to be epic politics and war, with few female perspectives and none at all from outside the ruling elites, and not a lot of characterisation.
But I gave it another try, this time curbing my ambition and going only for the abridged University of California Press edition translated and edited by Moss Roberts, which cuts out about half of the material. I found this more digestible, though I still felt the need of maps to explain where the three contested kingdoms were in relation to any geography that I am familiar with. The main strand of narrative of the book is the rise to power of Cao Cao to displace the authority of the Han dynasty emperor (in the late second century of our era), but his power is restricted to the northern kingdom of Wei; after his death, his sons actually displace the Han heir from the throne, but their rule declines and eventually ends (in the mid-third century of our era). Wu and Shu, the other two kingdoms which have split off from the Han realm, alternatively fight Cao Cao and each other, but the core narrative seems to me to be in the north. It's a detailed study of the use and abuse of military and political power, drawing on Chinese philosophy and Sun Tsu (who is repeatedly quoted, with approval), and the moral we are supposed to draw is taht integration must follow disintegration. I didn't feel equipped to engage with it as I would have liked, because of my lack of familiarity with the core material. I think there may be a market for a Three Kingdoms for Dummies edition, with maps and family trees.
One thing that struck me, both on this reading and my previous effort, was the role of magic and especially ghosts in the story. The unjustly executed become unquiet dead, haunting those who persecuted them, often with direct physical consequences, such as frightening generals and kigns to death. I don't think this makes the story a fantasy any more than the Lovejoy books are fantasy due to their protagonst's supernatural ability to detect antiques – rather less so, if anything, given that it's presented as a normal part of the world of the Three Kingdoms, but it's worth noting.