It makes sense if you consider the fact that the Old Testament points to Christ; and as an eternal being, Jesus was present for all the Old Testament events, He was there. So where does one find Him? Here are some allusions to the Trinity:
26Then God said, "Let Us make man in Our image, according to Our likeness; and let them rule over the fish of the sea and over the birds of the sky and over the cattle and over all the earth, and over every creeping thing that creeps on the earth."
Genesis 1:26 (NASB)
22Then the LORD God said, "Behold, the man has become like one of Us, knowing good and evil; and now, he might stretch out his hand, and take also from the tree of life, and eat, and live forever"
Genesis 3:22 (NASB)
7"Come, let Us go down and there confuse their language, so that they will not understand one another's speech."
Genesis 11:7 (NASB)
These are just three from Genesis, but you can see the concept of the plurality within the essence of God from the very beginning of the Bible. Some might argue that this is the "royal" we; that God is speaking royally, as human kings would do. However, there are no passages of such royal speaking when human kings are depicted in the Bible, so i do not place any stock in that argument.
Can you think of other Old Testament passages that speak to this? Do you agree that these refer to a plurality in God, if not specifically of the Trinity itself?
Please let me know what you think!