God has always been who God is. He never "became"a triune being. He has always been Father, Son, and Holy Spirit. That is who He is.
Before Rome, there existed an understanding of a triune God. It was not invented by the Roman Catholic church. Rome is where much corruption came into that church, but the doctrine of the Trinity was not one of the corruptions, and it is not what the reformers were addressing.
If God is a triune God, then indeed that is the God Israel worshiped. The teaching of His triune existence was not as fully revealed in the OT as it is in the NT, but it is shown in shadows (the Rock, the prophecies, and revealed in the incarnation with the person and work of Jesus and the explanation of these shadows and prophecies by Jesus Himself and later his appointed apostles. It is seen in many places in the NT but here are a couple of examples.
1 Cor 10:4 And all drank the same spiritual drink. For they drank from the spiritual Rock that followed them, and the Rock was Christ.
Col 2: 1-16 which concludes in vers 17. These are a shadow of the things to come, but the substance belongs to Christ.
John 17:3 and 1 Cor 8:6 are not refuting the Trinity. Faith in Jesus as Son of God (1 John 4:15) is the only way to be reconciled to God. John 14:6 Jesus answered. "I am the way and the truth and the life. No one comes to the Father except through me." If Jesus had been refuting his deity he would not have said in the same prayer, 5. And now, Father, glorify mein your own presence with the glory that I had with you before the world existed.
If 1 Cor 8:6 (yet for us there is one God, the Father, from whom are all things and for whom we exist, and one Lord, Jesus Christ, through whom are all things and through whom we exist.) Paul would not have said the very same thing of the Father that he says of Jesus.