The old testament people had been interacting with all the three persons of the God Head.
God The Father of the nation of Israel / Individuals
Deut 32:6, Isa 63:16, Jeremiah 3:4, Jeremiah 3:19 , Mal 1:6, 2 Sam 7:14 ,1 Chron 17:13 Psalm 68:5
The Angel of the Lord - Christophany
Exodus 3:2-6, Genesis 16:7, Judges 2:1-2, Judges 13:16-22, Zechariah 3:1-2, Zechariah 1:12-13, Psalms 34:7
Genesis 1:2, Job 33:4, Psalm 104:30,Nehemiah 9:20,Job 32:8, Psalm 51:11,Psalm 139:7,Exodus 31:3
One old testament verse that supports Trinity is Isa. 48:16-17 "Come near to Me, hear this: I have not spoken in secret from the beginning; from the time that it was, I was there. And now the Lord GOD and His Spirit have sent Me."Thus says the LORD, your Redeemer, the Holy One of Israel: "I am the LORD your God, who teaches you to profit, who leads you by the way you should go.
Lets examine this again because this theme occurs over and over Isaiah 48:16:“From the beginning from the time that it was, I was there: (1st person) and now the Lord GOD (2nd person) and His Spirit, (3rd person) hath sent me (1st person).”
The Trinity Is Foreshadowed In Benedictions
The Trinity is also foreshadowed in the triple benediction of Numbers 6:24-27. The LORD bless you and keep you; the LORD make his face shine upon you and be gracious to you; the LORD turn his face toward you and give you peace. So they will put my name on the Israelites, and I will bless them.
The doxology of Isaiah 6:3 also gives hint of the Trinity. And they were calling to one another: Holy, holy, holy is the LORD Almighty; the whole earth is full of his glory (Isaiah 6:3).