Albert Barnes Commentary Matthew 18:16

"But if he hear [thee] not, take with thee one or two more, that at the mouth of two witnesses or three every word may be established." — Matthew 18:16 (ASV)

If he will not hear thee, etc. That is, if he rejects or abuses you, or will not be persuaded by you, and will not reform.

Take one or two more. The purpose of taking them seems to be:

1. that he might be persuaded to listen to them (Matthew 18:17). They should be persons of influence or authority; his personal friends, or those in whom he could trust.
2. That they might be witnesses of his conduct before the church (Matthew 18:17). The law of Moses required two or three witnesses (Deuteronomy 19:15; 2 Corinthians 13:1; John 8:17).