小三 is translated to "the third person" or "the mistress" in English. This term originated from Chinese culture, where it is used to refer to a woman who is involved in an extramarital affair with a married man. Here are some examples of how you can use it in a sentence:

John's wife found out about his 小三 and was very angry.

The 小三 was seen out in public with John, causing a scandal.

Many people frown upon the idea of having a 小三 in a marriage.

John's 小三 was the reason for his divorce.

Please note that this term has a negative connotation and may be considered offensive by some people. It is best to avoid using it unless you are certain that it is appropriate in the context.