Luo Guanzhong's 14th century historical Chinese novel Romance of the Three Kingdoms is a much-loved subject of cultural adaptations, particularly in the east where it's an obvious source of rich story-telling material. However, I can only imagine that its author would be spinning in his grave at the knowledge that his magnum opus is currently being aired as an anime with most of its prominent characters replaced by cute and colourful girls. Come to think of it, he'd probably be confused by the mere notion of his story streaming around the world via the Internet, so perhaps we shouldn't worry about what he thinks of the whole endeavour too much at all...
Anyhow, Otome Tairan is actually the third season of the Koihime Musou anime, which itself started its life as an adult visual novel (soon to be released outside of Japan by MangaGamer, incidentally) upon which the anime is very loosely based; a fact which probably gives you a bit of an idea as to exactly how much it deviates from its historical Chinese source material.
Otome Tairan dispenses with any preamble and launches straight into this third series without so much as a glance over its shoulder at what came before, making for a slightly confusing spectacle if you haven't watched either prior series of Koihime Musou as characters in disparate scenes flood the screen without so much as a hello. There are plenty of those characters too, with the opening episode in particular skipping here and there, while the third episode takes us off to an entirely different tale to that which we'd be following from the first two instalments.
Despite its source material, Otome Tairan's focus is very much on the light-hearted, from a story of dieting and subsequent binge eating gone wrong through to a quest to find belly button fluff from an elephant as part of a cure for a woman who has grown cat ears (I'm not making this up, I promise you). Even when the story does get a little more serious, as per episode three's tale of a bodyguard named Ryou and her growing friendship with the woman she serves, it's still played for laughs more than it is for drama, while both of these elements are over-ridden by the need for the show's characters to all be nauseatingly cute as often as possible.
In fact, if I had to sum up Otome Tairan in one word it would be "saccharine" - All of the characters are as predictably sweet as you might expect in terms of both personality and appearance, serving to be fawned over by fans rather than offering up much excitement or plot development while covering as many physical characteristics in the database of "stuff otaku like" as possible. Never mind the world of war and evil around which the series is set, Otome Tairan wants to make you smile and enjoy the company of cute girls, while also dishing out the odd bit of (frequently censored) fan service.
With all of this in mind, Otome Tairan isn't the kind of series you can just dip in to - You simply have to watch the first couple of series to bring yourself up to speed with what's going on here and to help you figure out who's who. If you've already watched the rest of Koihime Musou then you won't need to be told to watch this third season, but if you've never sat down in front of it before then Otome Tairan isn't the best of starting points, while the franchise as a whole appears to be a particularly vacuous case of style over substance, with the style in question being "look at all these girls!".
At the time of writing, Shin Koihime Musou - Otome Tairan can be viewed in streaming format from Crunchyroll - The first three episodes are currently available to be viewed on the site, with a new episode released every Friday.