The concept of the Trinity—the belief in one God in three persons (Father, Son, and Holy Spirit)—is primarily a Christian doctrine that is more explicitly developed in the New Testament. However, some Christians find references or foreshadowings of the Trinity in the Old Testament. Here are a few examples that are often cited:

1. Genesis 1:26

KJV 26 And God said, Let us make man in our image, after our likeness: and let them have dominion over the fish of the sea, and over the fowl of the air, and over the cattle, and over all the earth, and over every creeping thing that creepeth upon the earth.

CEV 26 God said, “Now we will make humans, and they will be like us. We will let them rule the fish, the birds, and all other living creatures.”

• Verse: "Then 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.'"
  
• Explanation: The use of "Us" and "Our" suggests a plurality within God. Some Christians interpret this as a hint of the Trinity, though it is not explicit.

2. Genesis 18:1-2

KJV 1 And the Lord appeared unto him in the plains of Mamre: and he sat in the tent door in the heat of the day; 2 And he lift up his eyes and looked, and, lo, three men stood by him: and when he saw them, he ran to meet them from the tent door, and bowed himself toward the ground,

CEV 1 One hot summer afternoon while Abraham was sitting by the entrance to his tent near the sacred trees of Mamre, the Lord appeared to him. 2 Abraham looked up and saw three men standing nearby. He quickly ran to meet them, bowed with his face to the ground,

• Verses: "Now the LORD appeared to him by the oaks of Mamre, while he was sitting at the tent door in the heat of the day. When he lifted up his eyes and looked, behold, three men were standing opposite him; and when he saw them, he ran from the tent door to meet them and bowed himself to the earth."
  
• Explanation: The appearance of three men to Abraham is sometimes seen as a foreshadowing of the Trinity.

3. Isaiah 6:8

KJV 8 Also I heard the voice of the Lord, saying, Whom shall I send, and who will go for us? Then said I, Here am I; send me.

CEV 8 After this, I heard the Lord ask, “Is there anyone I can send? Will someone speak for us?” “I'll go,” I answered. “Send me!”

• Verse: "Then I heard the voice of the Lord, saying, 'Whom shall I send, and who will go for Us?' Then I said, 'Here am I. Send me!'"
  
• Explanation: The shift from "I" to "Us" in this verse is interpreted by some Christians as indicating a plurality within the Godhead.

4. Isaiah 9:6

KJV 6 For unto us a child is born, unto us a son is given: and the government shall be upon his shoulder: and his name shall be called Wonderful, Counsellor, The mighty God, The everlasting Father, The Prince of Peace.

CEV 6 A child has been born for us. We have been given a son who will be our ruler. His names will be Wonderful Advisor and Mighty God, Eternal Father and Prince of Peace.

• Verse: "For a child will be born to us, a son will be given to us; And the government will rest on His shoulders; And His name will be called Wonderful Counselor, Mighty God, Eternal Father, Prince of Peace."
  
• Explanation: This prophecy about the Messiah includes titles that reflect different aspects of God's nature, which some Christians see as indicative of the Trinity.

5. Psalm 2:7

KJV 7 I will declare the decree: the Lord hath said unto me, Thou art my Son; this day have I begotten thee.

CEV 7 I will tell the promise that the Lord made to me: “You are my son, because today I have become your father.

• Verse: "I will surely tell of the decree of the LORD: He said to Me, 'You are My Son, Today I have begotten You.'"
  
• Explanation: This verse is seen as a reference to the relationship between God the Father and God the Son.

6. Psalm 110:1

KJV 1 The Lord said unto my Lord, Sit thou at my right hand, until I make thine enemies thy footstool.

CEV (A psalm by David.) 1 The Lord said to my Lord, “Sit at my right side, until I make your enemies into a footstool for you.”

• Verse: "The LORD says to my Lord: 'Sit at My right hand Until I make Your enemies a footstool for Your feet.'"
  
• Explanation: Jesus Himself quotes this verse in the New Testament (Matthew 22:44

  KJV 44 The Lord said unto my Lord, Sit thou on my right hand, till I make thine enemies thy footstool?

  CEV 44 ‘The Lord said to my Lord: Sit at my right side until I make your enemies into a footstool for you.’

  ), interpreting it as a reference to His divine nature and His relationship with God the Father.

7. Proverbs 30:4

KJV 4 Who hath ascended up into heaven, or descended? who hath gathered the wind in his fists? who hath bound the waters in a garment? who hath established all the ends of the earth? what is his name, and what is his son's name, if thou canst tell?

CEV 4 Has anyone gone up to heaven and come back down? Has anyone grabbed hold of the wind? Has anyone wrapped up the sea or marked out boundaries for the earth? If you know of any who have done such things, then tell me their names and their children's names.

• Verse: "Who has ascended into heaven and descended? Who has gathered the wind in His fists? Who has wrapped the waters in His garment? Who has established all the ends of the earth? What is His name or His son's name? Surely you know!"
  
• Explanation: The reference to God's Son is seen by some as a hint of the Trinity.

8. Isaiah 48:16

KJV 16 Come ye near unto me, hear ye this; I have not spoken in secret from the beginning; from the time that it was, there am I: and now the Lord God, and his Spirit, hath sent me.

CEV 16 Come closer and listen! I have never kept secret the things I have said, and I was here before time began. By the power of his Spirit the Lord God has sent me

• Verse: "Come near to Me, listen to this: From the first I have not spoken in secret, From the time it took place, I was there. And now the Lord GOD has sent Me, and His Spirit."
  
• Explanation: This verse mentions the Lord GOD, Me (the speaker, often interpreted as the Messiah), and His Spirit, which some see as a reference to the Trinity.

Summary

While the Old Testament does not explicitly teach the doctrine of the Trinity, certain passages are interpreted by Christians as foreshadowing or hinting at the triune nature of God. These interpretations are generally made in light of the fuller revelation of the Trinity found in the New Testament.