Hear, O Israel, Adonai Eloheinu Adonai is one. These three are one. How can the three Names be one? Only through the perception of faith; in the vision of the Holy Spirit, in the beholding of the hidden eye alone.…So it is with the mystery of the threefold Divine manifestations designated by Adonai Eloheinu Adonai” three modes which yet form one unity

This is from A Jewish book Zohar II:43b