Many Scriptures confirm these three witnesses

The Father - The Son - The Holy Spirit

The Father:

Jeremiah 29:23

For they have done outrageous things in Israel; they have committed adultery with their neighbors’ wives and in my name have spoken lies, which I did not tell them to do. I know it and am a witness to it,” declares the LORD.

Malachi 3:5

“So I will come near to you for judgment. I will be quick to testify against sorcerers, adulterers and perjurers, against those who defraud laborers of their wages, who oppress the widows and the fatherless, and deprive aliens of justice, but do not fear me,” says the LORD Almighty.

John 5:31-37

31 “If I testify about myself, my testimony is not valid.

32 There is another who testifies in my favor, and I know that his testimony about me is valid.

33 “You have sent to John and he has testified to the truth.

34 Not that I accept human testimony; but I mention it that you may be saved.

35 John was a lamp that burned and gave light, and you chose for a time to enjoy his light.

36 “I have testimony weightier than that of John. For the very work that the Father has given me to finish, and which I am doing, testifies that the Father has sent me.

37 And the Father who sent me has himself testified concerning me. You have never heard his voice nor seen his form,

Romans 1:9

God, whom I serve with my whole heart in preaching the gospel of his Son, is my witness how constantly I remember you

Hebrews 1:1-2

1 In the past God spoke to our forefathers through the prophets at many times and in various ways,

2 but in these last days he has spoken to us by his Son, whom he appointed heir of all things, and through whom he made the universe.

Hebrews 2:3-4

3 how shall we escape if we ignore such a great salvation? This salvation, which was first announced by the Lord, was confirmed to us by those who heard him.

4 God also testified to it by signs, wonders and various miracles, and gifts of the Holy Spirit distributed according to his will.