The Messiah was indeed mentioned in the Old Testament, but NEVER was the Messiah defined as the Son of God.
There is no mention whatever of any sort of Trinity in the Old Testament. None at all, not even a suggestion.
There is no mention that the Messiah is supposed to be a son of God, either. He is pretty much defined as a human leader from the house of David with perhaps less adventurous gonads than David, and better organizational skills so as to defeat the Jews' enemies.
And what is it with this Holy Spirit stuff, anyway? Humans come in two genders, and all three members of the Trinity are defined as masculine.
Jesus has a whole bunch of books of the Bible all for himself, but then some wacky theologians seem not to have appreciated a dual god and thought that a three-headed deity made more sense to them.
The Holy Mother Church admits that this is a wacky concept, by calling it by the name it gives all forms of wacky Catholicism: a "Mystery of the Faith".
Matthew 16:16
As far as I know the trinity isn't mentioned as such in the New Testament either.
It is a way of understanding what Jesus was telling us about why he had to leave for a while.
Exodus 3:13
Moses learns to refer to God as "I Am".
Psalm 51:11
Isaiah 63:10-11
Holy Spirit gets mention
Isaiah 9:6
Messiah gets mention, is this just three names for the same person , or three aspects of a larger whole?
I don't feel qualified to determine. But the Trinity is there , but I don't consider it important to determine exactly what it is. In no scripture does acknowledgment of the Trinity become an important part of salvation , in several places the unity of a single God is emphasized , so no matter what we think it means , it cannot be construed as being three gods.
Moving to the New Testament.....
Matthew 1:20
Holy Spirit gets mention again
Jesus gets called Son of God ...
Matthew 4:3
By Satan...
Matthew 8:29
By heard of demons...
Matthew 27:54
Centurions...
Luke 1:35
Angels...
Luke 22:70
Crowds...
Mark 9:7
Huge voice from the clouds....
John 5:25 And himself....
I tell you the truth, a time is coming and has now come when the dead will hear the voice of the Son of God and those who hear will live.
John 19:7
This causes a problem with the established religious authority.
So the idea becomes the foundation of Christianity.
Acts 9:20
Saul in Damascus and Jerusalem , Saul spent several days with the disciples in Damascus. At once he began to preach in the synagogues that Jesus is the Son of God.