"menage à trois"
It's a literary reference.
In literature, it is when there is a man, a woman, and a lover (in various patterns, but the most common is that the lover is the woman's).
A "love triangle", we'd call it now, but the French call it a "menage à trois".
Most literally, it means something like "three in the house".
The French, however, do not have the same connotation for "an orgy between three people" that it has in English. At least, the one that explained that to me had never heard of it.