“小三”最常用的表达是"mistress"， "lady friend"，"the other woman"，和"home wrecker"。当然考虑到小三不一定都是女生，男小三也可以用"the other man"或“home wrecker”来表达。
mistress 情妇
一个男人(通常是已婚男人的)mistress是指与他有固定性关系但不是他妻子的女人
例：
Edward VII and his mistress, Lillie Langtry
爱德华七世和他的情妇莉莉‧兰特里
He has a mistress in Paris.
他在巴黎有个情妇。
lady friend
通常作为委婉语使用，或者在情况不明时作为一种保守的表达
the female partner in an intimate especially an illicit relationship
亲密关系中的女性伴侣，尤指不正当关系中的女性伴侣
the other woman/man
与已婚男人发生关系的人
例：
Was she no longer satisfied being the other woman?
她不再满足于当第三者了吗?
home wrecker
/ˈhəʊm.rek.ər/
someone who is responsible for breaking up a marriage or relationship
破坏别人家庭(或恋爱关系)的人
例：
He denied he was a homewrecker, despite his fling with Sarah being blamed for the collapse of her marriage.
他否认自己是一个破坏别人家庭的人，尽管他与萨拉的风流韵事被指责是导致她婚姻破裂的原因。
Her reputation as a potential homewrecker seems ludicrously misplaced.
她可能破坏别人家庭的名声似乎大错特错了。
“出轨”就是我们俗称的“脚踏两条船”，也即“劈腿”，可以用两种英文句式来表达：
He is having an affair. 他有外遇了。
Does he cheat on his wife? 他是不是出轨了?
或者用一个动词
two-time
vt. 欺骗;劈腿;对(情人或爱人)怀有二心(作为动词使用)
例：
Are you sure he's not two-timing you?
你肯定他没有背着你另有所爱?
cheater
背叛者，渣男
例：
You've been dating that girl recently and you're a total cheater.
你最近一直跟那个女孩约会，你真是个彻头彻尾的渣男。
playboy
花花公子
例：
He is such a playboy. How many times has he hurt you?
他是个十足的花花公子。他伤害过你多少次了?
