最近因為某部電視劇讓 "小三" 話題發燒
Henry has a mistress in China.
have an affair / have affairs 有外遇
John denied that he was having an affair.
cheat on sb. / be unfaithful to sb. 對某人不忠
Did you cheat on me?
Ian is unfaithful to his wife.
two-time (v.) 劈腿
Are you sure he is not two-timing you? (Macmillan dictionary)
two-timer (n.) 劈腿者
Diana broke up with Ryan because he is a two-timer.