Judaism has the belief that God has a triune nature. The rabbis since have imposed pagan ideas upon the basic idea.
“In the Zohar (the classic text of Kabbalah) and other Jewish sources, we find that there are three (manifestations of Godliness) which are essentially One: God, Israel and the Torah.” --Rabbi Yitzchak Ginsburgh
Rather than acknowledge that God is triune, they make themselves equal to God.
Before it was perverted, the basic idea comes from the gematria of the word for father אב which has the numerical value of 3. The word for son is בנ and the word for rock is אבנ (the father and the son). The stone that was split is a symbol of the Father and Son being split at the cross when Jesus cried “Why have you foresaken me?” It is a similar picture/prophecy as the water which was parted, and the veil which was torn.
God said that he was their stone (Ge 49.24). The law was written upon the stone (Ex 31.18) so they were the tablets of the father-son.