Maybe the three witnesses for us on earth are what the Church refers to as the Baptism of desire (My soul is thirsting for the living God); the Baptism of water, as we know it; the Baptism of blood (Martyrdom, like the Holy Innocents suffered when Jesus was a baby) that Fr. Mitch Pacwa speaks of sometimes. Just a thought.
From 1 John Chapter 5.
Faith is Victory over the World.
Everyone who believes that Jesus is the Christ is begotten by God, and everyone who loves the father loves [also] the one begotten by him.
In this way we know that we love the children of God when we love God and obey his commandments.
For the love of God is this, that we keep his commandments. And his commandments are not burdensome,
for whoever is begotten by God conquers the world. And the victory that conquers the world is our faith.
Who [indeed] is the victor over the world but the one who believes that Jesus is the Son of God?d
This is the one who came through water and blood,* Jesus Christ, not by water alone, but by water and blood.
The Spirit is the one that testifies, and the Spirit is truth. So there are three that testify,(some translations say three witnesses) the Spirit, the water, and the blood, and the three are of one accord.
If we accept human testimony, the testimony of God is surely greater.
Now the testimony of God is this, that he has testified on behalf of his Son.
Whoever believes in the Son of God has this testimony within himself.
Whoever does not believe God has made him a liar by not believing the testimony God has given about his Son.
And this is the testimony: God gave us eternal life, and this life is in his Son.
Whoever possesses the Son has life; whoever does not possess the Son of God does not have life.