Was the concept of Godhead (one God, but multiple persons) known to Judaism prior to the advent of Christianity? Were there Jews that held a Binitarian, Trinitarian or similar "Godhead" theology as a result of studying and interpreting the Torah, long before Christianity came on the scene?
The phrase "one God in multiple persons" is not technical at all, but rather my personal way of expressing the idea with words. For a more technical definition, please see the following quotes from Wikipedia:
Godhead (or godhood) refers to the divinity or substance (ousia) of the Christian God, especially as existing in three persons — the Father, Son, and Holy Spirit.
Binitarianism is a Christian theology of two persons, personas, or aspects in one substance/Divinity (or God). Classically, binitarianism is understood as a form of monotheism—that is, that God is absolutely one being—and yet with binitarianism there is a "twoness" in God, which means one God family. The other common forms of monotheism are "unitarianism", a belief in one God with one person, and "trinitarianism", a belief in one God with three persons.
The Christian doctrine of the Trinity (Latin: Trinitas, lit. 'triad', from Latin: trinus "threefold") holds that God is one God, but three coeternal and consubstantial persons: the Father, the Son (Jesus Christ), and the Holy Spirit. The three persons are distinct, yet are one "substance, essence or nature" (homoousios). In this context, a "nature" is what one is, whereas a "person" is who one is.