Going off of a previous question [asking about threesomes in Islamic society] (https://www.reddit.com/r/AskHistorians/comments/3ie2ho/in_muslim_societies_where_polygamy_was_common_did/), where it seems that wives were kept very separated and the primary wife would be unlikely to allow it, I was wondering about the situation in more Eastern courts, such as China, Japan, Korea, or Indochina. It is my understanding that in those places a monarch would often have many wives, including a primary one, but I imagine the social mores would be a little different.
First off, apologies for the late response.
I study the Korean court during the later years of the Joseon dynasty, roughly from the late 16th to early 19th centuries. Information on the sexual life of Joseon kings is generally lacking in our sources. Sex was too much of a taboo subject for the details of it to be explicitly discussed in the context of the court or, more widely speaking, the writers of the elite class (an interesting thing to note is that in 1519, there was some talk of having female chroniclers record courtly events occuring at night as well, under arguments such as having all actions recorded for posterity would encourage the monarch to be more righteous and moral; the king of the time refused, claiming that women were not versed well enough in Classical Chinese and that it would be too much of a hassle to find women who were morally upright enough to take the job). For all we know, Joseon kings were having threesomes with their concubines every night, we just have no records or evidence.
Chinese erotic literature do occasionally feature threesomes and other forms of group sex. However, that never meant they were socially acceptable, any more than everything in modern porn is acceptable today. Far from it. To quote Sex in China: Studies in Sexology in Chinese Culture by Fang Fu Ruan:
The attempt to invoke the thrill of the forbidden may also have motivated depictions of incestuous, pedophilic, premarital, and adulterous (with or without spousal consent) relationships, and group sex ranging from threesomes to full-blown orgies.
A more important thing that you should realize is that Chinese elite men, and by extension elite men in many other parts of East Asia (this was hardly universal in the region, as elite men of the later Goryeo dynasty in Korea had two wives with equal rights, although they wouldn't be having any threesomes considering that the second wife was always much younger than the first wife), simply did not simultaneously have multiple wives. They had one wife and multiple concubines, in the same way as European rulers had mistresses, and concubines were in every way a step below a wife just as a mistress was uncomparable to a full wife. Threesomes between the emperor and his wives would not have been possible, because the emperor had only one wife.