三人行必有一小三
[sān rén xíng bì yŏu yī xiăo sān]
Derived from the Confucian saying, “Whenever three are walking together, one will surely be capable of teaching me something,” but it’s given an ironic twist here: 'Whenever there's a gathering of three people, there's inevitably going to be drama or conflict between them (e.g., a third wheel).' This refers to the presence of a problematic party in every triadic relationship or friendship group.