The Trinity - Father, Son, Holy Spirit - is one God, and many get tangled up. Many have published great books on the Trinity and others have tiny essays.
I have startled some with the unity of the Triune God, because the opening of John's Gospel also begins with the Trinity.
A good way to understand this truth of God is to include the Father, Son, and Spirit with the Father.
This opens our eyes in many ways, and point out a few examples. I am including Psalms.
Psalm 2:7 I will declare the decree: the Lord hath said unto me, Thou art my Son; this day have I begotten thee.
The KJV is very clear with two references to the Son of God (above). The NIV tends toward weakness rather than clarity.
The Psalms are a wonder in showing how the Word of God teaches the Trinity together and in clarifying those passages. People can see more clearly how the Trinity is both One and Three, from the very Beginning throughout the Old and New Testaments.
Not teaching the Lord Jesus in the Old Testament is an error that builds up in the New Testament. People who deny the Son and the Holy Spirit weaken and deny the unity of the Father, Son, and Spirit.