The Catechism in Questions & Answers

63. What references to the triune God are there in the Old Testament?

The first such reference to the trinity of God is found in Genesis 1: 26: “Let Us make man in Our image, according to Our likeness.” The use of the plural “Let Us” is a reference to the activity of God in several persons.
God appeared to Abraham in Mamre in the form of three men (cf. Genesis 18). This is understood as a reference to the trinity of God.
It is the same with the threefold blessing (“Aaronic blessing”), which Aaron spoke over the people of Israel (Numbers 6: 24-26).

“The Lord bless you and keep you; the Lord make His face shine upon you, and be gracious to you, the Lord lift up His countenance upon you, and give you peace.”
Numbers 6: 24-26