Many bible scholars teach that the Old testament offers clues to the Compound or Tri-Unity of God. What that means is that while there is complete recognition that God is "one" there is more than one (or compound) way in which God has revealed through different components and continues to reveal Himself to us. Some of these passages are quoted below.
The introduction of the compound unity of God is found beginning with the creation process.
In the beginning God created the heavens and the earth. And the earth was formless and void, and darkness was over the surface of the deep; and the Spirit of God was moving over the surface of the waters (Genesis 1:1-2). Here we are introduced to two different components of the compound unity of God. The first, God (Elohim a plural noun) and then the second, the Spirit of God (v'Ruach Elohim).
Later in the creation process we read "Let US make man in OUR own image, and OUR own likeness." Genesis 1:26
Other indication of the compound or tri-unity of God may include:
Numbers 6:24-26. Sometimes this passage is referred to as the Aaronic or Priestly Blessing.
24) The LORD bless you, and keep you;
25) The LORD make His face shine on you, And be gracious to you;
26) The LORD lift up His countenance on you, And give you peace.
When the priest (cohen) would offer this blessing (Num. 6:24-26) over the congregation, their hands would be held up in the air, with palms facing outward toward the congregation, thumbs touching and fingers spread apart on each hand in the shape of a v. This specific hand imagery was an attempt to reproduce the hebrew letter shin.
According to Jewish tradition, the significance of this priestly gesture is found in the effort to recreate the letter shin because this letter represented the first letter of the hebrew word "Shaddai" which is one of the names for God meaning "Almighty." Notice the three parallel lines connected by a perpendicular line across the base. From this, some find support for the conclusion of the three into one compound unity. This same Hebrew letter is on the face of the mezuzah, the little box attached to every Jewish doorpost. Inside each mezuzah is a miniature scroll containing the Shema: "Hear O Israel, the LORD is God, the LORD is one."
For other interesting information concerning this prayer please read Lessons From The Shema.
Another possible clue is found in Isaiah's vision of the Lord sitting on His throne,"high and lifted up" (6:1-13). Angelic beings offer up worship in triads: "Holy holy,holy is the Lord of hosts: The whole earth is full of His glory! (v.3). Isaiah responded with this memorable declaration: "For mine eyes have seen the King, the LORD of Hosts" (v.5). Of whom was Isaiah speaking? The Apostle John refers to Isaiah's vision of God's glory, and also of Israel's rejection of that revelation, and says Isaiah saw the glory of Jesus (John 12:39-41). Isaiah saw Jesus and spoke of Him!
The prophet Jeremiah may provide yet another clue when he elaborates on the designation of the Messiah as the Branch that grows from Jesse's roots (23:5-6). He says one of the Messiah's names is "The LORD Our Righteousness." This is in effect equating the name of the Messiah with God.
Isaiah 63:8-10, is another area of scripture that may reveal the compound unity of God.
1) God the Father, the one who called Israel "my people,"
2) The Angel of His Presence who saved and redeemed them.
3) The Holy Spirit (Ruach HaKodesh), whom they grieved by their disobedience.
Sometime a Compound Unity is also referred to as a Complex Unity.
Check our these related articles that help to support the Divinity of the Messiah:
Christian Jew Foundation, Messianic Perspectives September-October 2001.