1 John 5:6 This is he that came by water and blood, even Jesus Christ; not by water only, but by water and blood. And it is the Spirit that beareth witness, because the Spirit is truth.
7 For there are three that bear record in heaven, the Father, the Word, and the Holy Ghost: and these three are one. 8 And there are three that bear witness in earth, the Spirit, and the water, and the blood: and these three agree in one. 9 If we receive the witness of men,
the witness of God is greater: for this is the witness of God which he hath testified of his Son. KJV
What is John talking about? Here below is John the Baptist's testimony as reported by the apostle John;
John 1:31 And I knew him not: but that he should be made manifest to Israel, therefore am I come baptizing with water. 32 And John bare record, saying, I saw the Spirit descending from heaven like a dove, and it abode upon him. 33 And I knew him not: but he that sent me to baptize with water, the same said unto me, Upon whom thou shalt see the Spirit descending, and remaining on him, the same is he which baptizeth with the Holy Ghost. 34 And I saw, and bare record that this is the Son of God.
Now look at confirmation from the gospel of Mattew of the 3 Witnesses of the Son as God being sent from Heaven.
Matthew 3:15 And Jesus answering said unto him, Suffer it to be so now: for thus it becometh us to fulfil all righteousness. Then he suffered him. 16 And Jesus, when he was baptized, went up straightway out of the water: and, lo, the heavens were opened unto him, and he saw the Spirit of God descending like a dove, and lighting upon him: 17 And lo a voice from heaven, saying, This is my beloved Son, in whom I am well pleased.
Jesus is standing there after being water baptized, that is one Witness, the Holy Spirit alighting on Him like a dove as the 2nd Witness, and the Father speaking from Heaven regarding His Son as God is the third Witness.
No one person can serve as three witnesses by His words, and so there are 3 Witnesses within the One God for how as the Triune God, God establishes a testimony and how God judges any one.
Deuteronomy 17:6 At the mouth of two witnesses, or three witnesses, shall he that is worthy of death be put to death; but at the mouth of one witness he shall not be put to death.
Deuteronomy 19:15 One witness shall not rise up against a man for any iniquity, or for any sin, in any sin that he sinneth: at the mouth of two witnesses, or at the mouth of three witnesses, shall the matter be established.
Jesus reaffirmed and taught it in the N.T. when correcting any brother in error or in a trespass.
Matthew 18:15 Moreover if thy brother shall trespass against thee, go and tell him his fault between thee and him alone: if he shall hear thee, thou hast gained thy brother. 16 But if he will not hear thee, then take with thee one or two more, that in the mouth of two or three witnesses every word may be established.
Even for establishing a true witness.
John 8:17 It is also written in your law, that the testimony of two men is true.
So for everything that Jesus has said, the Father & the Holy Spirit are each separate Witnesses from Jesus in making what Jesus has said as a true witness.
Paul reaffirmed and taught the same thing when establishing a testimony or judging any one since it cannot be done by one person.
2 Corinthians 13:1 This is the third time I am coming to you. In the mouth of two or three witnesses shall every word be established.
1 Timothy 5:19 Against an elder receive not an accusation, but before two or three witnesses. 20 Them that sin rebuke before all, that others also may fear.
When we see how God created man, He did it in creating man per the Word of God's request to the Father in being plural and yet when in agreement to that request, as the One God, the Word of God created man after His image and after His likeness thus establishing a word or a testimony in creation.
Genesis 1:26
And God said, Let us make man in our image, after our likeness: and let them have dominion over the fish of the sea, and over the fowl of the air, and over the cattle, and over all the earth, and over every creeping thing that creepeth upon the earth.
27 So God created man in his own image, in the image of God created he him; male and female created he them.
We can see the same when God judged the people at the tower of Babel as the Word of God made the request to the Father & as the Triune God, to go down and scatter the people in confusing them by giving them different languages.
Genesis 11:5 And the Lord came down to see the city and the tower, which the children of men builded. 6 And the Lord said, Behold, the people is one, and they have all one language; and this they begin to do: and now nothing will be restrained from them, which they have imagined to do. 7 Go to
, let us go down, and there confound their language, that they may not understand one another's speech. 8 So the Lord scattered them abroad from thence upon the face of all the earth: and they left off to build the city.
The Hebrew word " Elohim " for God in the English language is plural and so from the very beginning since the creation account, God has been a Triune God in how as the One God He had established a word in creation & had judged any one.