The Father, Son, and Holy Spirit comprise the one God. That God is triune from eternity is attested by the self-revelations of God within the history of salvation, which makes clear that the Father, the Son, and the Holy Spirit have existed, created, acted, and sustained from the beginning.
In the old covenant God predominantly revealed Himself as God, the Father, whereas the activity of the Son and the Holy Spirit was still largely concealed from mankind. From the perspective of the New Testament, Apostle Paul declares that the Son of God was already present when the people of Israel made their way through the desert (1 Corinthians 10: 4). Beyond that, both Mark 12: 36 and Hebrews 3: 7 state that the Holy Spirit already spoke in the old covenant.
The incarnation, death, and resurrection of the Son of God, as well as the sending of the Holy Spirit, allow believers to recognise God as triune. In John 16: 13-15, Jesus Christ underlines the workings of the trinity of God: that which belongs to the Son also belongs to the Father, and that which the Holy Spirit declares, He takes from the Father and the Son.
The triune God is one God in the fellowship of Father, Son, and Spirit. He seeks to make this fellowship of His accessible to mankind.