Diànshìjù lǐ cháng yǒu xiǎosān pòhuài biéren jiātíng de qíngjié.
TV dramas are full of plots about a mistress breaking up someone else's family.
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Colloquial and scornful, and aimed at the outsider rather than the unfaithful spouse: 三 counts the third person in a couple. More neutral words are 第三者 'third party' and 情妇 'mistress'.
usage
It can also be short for 小学三年级 'third grade of primary school', just as 小一 is the first grade.
