אֶחָד is one......
Deuteronomy 6:4 (NIV)
4 Hear, O Israel: The Lord our God, the Lord is one אֶחָד
Old Testament says God is one.
One God in one Person?
God is one, can be translated as "Joined One," please see:
Ezekiel 37:15–17 (NIV)
15 The word of the Lord came to me: 16 “Son of man, take aאֶחָד stick of wood and write on it, ‘Belonging to Judah and the Israelites associated with him.’ Then take another stick of wood, and write on it, ‘Belonging to Joseph (that is, to Ephraim) and all the Israelites associated with him.’ 17 Join them together into oneאֶחָד stick so that they will become oneאֶחָד in your hand.
Two sticks joined into one, and became one!
God the Father joined with God the Son and joined with God the Holy Spirit, they became one God.
Our God is One God in Three Persons, Triune God.