Three people (sometimes more) are connected via multiple romantic relationships or unrequited romantic aspirations in a so called "love triangle" ("love mesh" more generally). For example: A loves B loves C loves A, or A and B are friends and both lovers of C, etc.

Notes:
To be applicable, some sort of interpersonal relationship should be present for all pairs of people in the triangle. In particular, the theme is not applicable when a spouse takes a lover unless the other spouse is also well acquainted, for example friends, with the lover in question.

References:
https://en.wikipedia.org/wiki/Love_triangle