The Hebrews were not trinitarian. It would be an improper eisegesis to suggest that they were. Trinitarianism is something that evolved or developed in Christian theology.
The ancient Hebrews were monotheistic, but even that developed over time. God was Abram's God. And then the God of his family. And then the God of a people. And then the God of a land. And then the God of all the earth. As their experiences with God developed - from the whisper in Abraham's ear to his shouts to Moses on the mountain - so did their understanding of him. God did not change, but the belief of his faithful people did.
By the time Jesus comes on the scene, fierce monotheism is the only option. "Hear, oh Israel, the Lord your God is one." There is one God, and all others are man-made imposters. That one God is an omnipresent spirit who judges and directs and decrees. But ... Jesus is doing miracles. And he is teaching with authority. And his is forgiving sins. Those are things only God can do. And then he says, "I am." Could he be?
Upon his resurrection, he is no longer just Rabbi; he is Lord, a title of God. By his faithful followers, he has been recognized to be the physical representation of their one invisible God. He is distinct from the Father, for they have heard his voice in the presence of Jesus the Son, but they are equal. Their experience is moving them away from the fierce monotheism of their youth. They are not abandoning it, but their understanding of what God is really like is evolving.
40 days after he returned to his Father, the followers of Jesus have another evolutionary moment. God becomes present with them in a way they had not yet experienced. His Spirit, distinct from the Father and the Son, promised by Jesus, blows upon them, causing them to speak in tongues, convicting them of sin, and giving birth to the church. Those are things only God can do. God is not just far away in heaven, nor just present in Jesus, but now he is alive within each of them.
It would take 300 years for them to be able to state with precision how their understanding of God had evolved. It was worth the wait. Here is what they came up with:
We believe in one God,
the Father, the Almighty,
maker of heaven and earth,
of all that is, seen and unseen.
We believe in one Lord, Jesus Christ,
the only Son of God,
eternally begotten of the Father,
God from God, light from light,
true God from true God,
begotten, not made,
of one Being with the Father;
through him all things were made.
For us and for our salvation
he came down from heaven,
was incarnate of the Holy Spirit and the Virgin Mary
and became truly human.
For our sake he was crucified under Pontius Pilate;
he suffered death and was buried.
On the third day he rose again
in accordance with the Scriptures;
he ascended into heaven
and is seated at the right hand of the Father.
He will come again in glory to judge the living and the dead,
and his kingdom will have no end.
We believe in the Holy Spirit, the Lord, the giver of life,
who proceeds from the Father [and the Son],
who with the Father and the Son is worshiped and glorified,
who has spoken through the prophets.
We believe in one holy catholic and apostolic Church.
We acknowledge one baptism for the forgiveness of sins.
We look for the resurrection of the dead,
and the life of the world to come. Amen.