Hi Shirley this is where the rule comes from.
The Watchtower Society use the scripture found in Deuteronomy chapter 9, verse 15, which says “No single witness should rise up against a man respecting any error or any sin, in the case of any sin that he may commit. At the mouth of two witnesses or at the mouth of three witnesses the matter should stand good.” They also like to use the scriptures in Matthew chapter 18, verses 15 and 16, which say “Moreover, if your brother commits a sin, go lay bare his fault between you and him alone. If he listens to you, you have gained your brother. (16) But if he does not listen, take along with you one or two more, in order that at the mouth of two or three witnesses every matter may be established.”
However this certainly does not work with sexual abuse as there are hardly any times when the abuse is actually witnessed by anyone.