So, while it is true the term itself does not appear in the Bible, the concept does. Here are some examples in the Old Testament.

1. In the beginning, God created the heavens and the earth. The earth was without form and void, and darkness was over the face of the deep. And the Spirit of God was hovering over the face of the waters. And God said, “Let there be light,” and there was light. (Genesis 1:1-3)

In the first words of the Bible, the word translated God is the Hebrew Elohim, the plural form of El. It means “the most high ones” or “the most powerful ones”. Nathan Holsteen and Michael Svigel state in their book Exploring Christian Theology, “Some theologians view the use of Elohim as a veiled reference to the three persons of the Trinity who will be revealed later in Scripture.” I am no Hebrew scholar, but it seems difficult to deny it is not at least a hint. Furthermore, you can see the phrase “Spirit of God” plainly right in the middle of the text. A less obvious hint is “God said”. This indicates God spoke, but the significance is God the Son (Jesus) is later referred to in the New Testament as “the Word”, through which all things were created (John 1:1-3; Colossians 1:16-17).

2. Then God said, “Let us make mankind in our image, in our likeness, so that they may rule over the fish in the sea and the birds in the sky, over the livestock and all the wild animals, and over all the creatures that move along the ground.” (Genesis 1:26)

3. Then I heard the voice of the Lord saying, ‘Whom shall I send? And who will go for us?’ (Isaiah 6:8)

In these passages, God is referring to himself in the plural forms of “us” and “our”. How can this be if God is not the Trinity?

Now you may say, “Jews were well-known for their robust monotheism. What about Deuteronomy 6:4 that states, the Lord is one?” To touch on this briefly I would like to quote from Great Doctrines of the Bible written by Martin Lloyd-Jones, “The particular emphasis …that there is only one true and living God… is because above everything else the children of Israel were called to proclaim the unity of God and the fact that there is only one God.”

4. I will proclaim the Lord’s decree: He said to me, “You are my son; today I have become your father. Ask me, and I will make the nations your inheritance, the ends of the earth your possession. You will break them with a rod of iron; you will dash them to pieces like pottery.” (Psalm 2:7-9)

5. Your throne, O God, will last for ever and ever; a scepter of justice will be the scepter of your kingdom. You love righteousness and hate wickedness; therefore God, your God, has set you above your companions by anointing you with the oil of joy. (Psalm 45:6-7)

6. The Lord says to my lord: “Sit at my right hand until I make your enemies a footstool for your feet.” (Psalm 110:1, quoted by Jesus in Matthew 22:41-46)

7. When there were no watery depths, I was given birth, when there were no springs overflowing with water; before the mountains were settled in place, before the hills, I was given birth, before he made the world or its fields or any of the dust of the earth. I was there when he set the heavens in place, when he marked out the horizon on the face of the deep, when he established the clouds above and fixed securely the fountains of the deep, when he gave the sea its boundary so the waters would not overstep his command, and when he marked out the foundations of the earth. Then I was constantly at his side. I was filled with delight day after day, rejoicing always in his presence, rejoicing in his whole world and delighting in mankind. (Proverbs 8:24-31)

8. Come near me and listen to this: “From the first announcement I have not spoken in secret; at the time it happens, I am there.” And now the Sovereign Lord has sent me, endowed with his Spirit. (Isaiah 48:16)

In these passages, we meet a plurality of divine persons, and in some of the references, one of whom is considered by the Jews to be the assured Messiah.