third_wheel

A third wheel is a person who feels unnecessary or unwelcome in a social situation, especially when accompanying a couple expressing affection or intimacy. This may lead to them feeling jealous, left out, or simply annoyed.

If the situation is sexual rather than humorous in nature, then it may be netorare instead. If the third wheel is intentionally spying on the intimate act, it may be voyeurism or eavesdropping instead.