Deuteronomy 17:6

17:6 At the testimony of two or three witnesses they must be executed. They cannot be put to death on the testimony of only one witness.

Deuteronomy 19:2

19:2 you must set apart for yourselves three cities in the middle of your land that the Lord your God is giving you as a possession.

Deuteronomy 19:15

19:15 A single witness may not testify against another person for any trespass or sin that he commits. A matter may be legally established only on the testimony of two or three witnesses.