3-person living arrangements.
I have not seen this term, "Trois Menage" used in any of these threads so far. In history, and classic literature, it is a living arrangement of three persons, nearly always a husband and wife and another man.
In the most traditional sense, the husband was considerably older than his wife, and could not perform sexually to satisfy her needs. So, a younger man was brought into the household, usually under the guise of being a gardner or some other servant, but his real role was to provide sexual services for the wife.
It was one of those arrangements that everyone understood, but no one criticized, since it was seen as being "necessary".
We still have such arrangements today, though they are rarely labeled as such, nor are they as obvious. I personally knew a young man who was hired by a small company, with one of his duties being to go to the company president's house twice a week to "perform" such duties as the boss's wife would need.
It was a blatant example of an older man providing a younger lover for his wife, and even paying the other man.
This is, in my extimation, an example of Polyamory!