Cheryl

The two or three witnesses requirement is stated in Deut 19:15

Deu 19:15 “A single witness shall not rise up against a man on account of any iniquity or any sin which he has committed; on the evidence of two or three witnesses a matter shall be confirmed.

Paul confirmed that this requirement of two or three witnesses is also to establish a matter:

Paul himself confirms a matter by the required two or three witnesses:

2Co 13:1 This is the third time I am coming to you. EVERY FACT IS TO BE CONFIRMED BY THE TESTIMONY OF TWO OR THREE WITNESSES.
2Co 13:2 I have previously said when present the second time, and though now absent I say in advance to those who have sinned in the past and to all the rest as well, that if I come again I will not spare anyone,

Jesus also confirmed that one testimony is not considered true:

Joh 5:31 “If I alone testify about Myself, My testimony is not true.
Joh 5:32 “There is another who testifies of Me, and I know that the testimony which He gives about Me is true.

The Pharisees questioned Jesus about the things that he was testifying about:

Joh 8:13 So the Pharisees said to Him, “You are testifying about Yourself; Your testimony is not true.”

Jesus answered that his testimony was not alone and therefore qualified by the “two or three witnesses” rule:

Joh 8:17 “Even in your law it has been written that the testimony of two men is true.
Joh 8:18 “I am He who testifies about Myself, and the Father who sent Me testifies about Me.”

So the Father is the second witness about the Son. John was also one who testified about Jesus and Jesus’ works were also considered another testimony:

Joh 5:36 “But the testimony which I have is greater than the testimony of John; for the works which the Father has given Me to accomplish–the very works that I do–testify about Me, that the Father has sent Me.
Joh 5:37 “And the Father who sent Me, He has testified of Me. You have neither heard His voice at any time nor seen His form.

The scriptures are still another testimony of Jesus:

Joh 5:39 “You search the Scriptures because you think that in them you have eternal life; it is these that testify about Me;

Paul tells us the importance of this repetition. The repetition is for our safety in Philippians 3:1 –

Php 3:1 Finally, my brethren, rejoice in the Lord. To write the same things again is no trouble to me, and it is a safeguard for you.