The truth is God is a " tri-unity".
Two invisible (Father and Spirit) and one visible Jesus/Yeshua.

"For ye know the grace of our Lord Jesus Christ, that, though, he was rich, yet for your sake he became poor, that ye through his poverty might be rich"
--- 2 Corinthians 8:9

The word "trinity" derived from the latin word trinitas means " threeness. Holy, Holy, Holy, is our God Almighty.

The Father even calls Jesus God.
"But unto the Son he saith, Thy throne, O God, is for ever and ever: a scepter of righteousness is the scepter of thy kingdom." (Hebrews 1:8)

The Old Testament was written long before the New Testament.
Simply put, "one" in the Bible does not always mean numerical quantity.
Depending on the scripture, "one" can often mean unity.