@Gihan Sorry I was not able to get back to you last night concerning some references to the Trinity in the OT. My time has been limited and unfortunately by reply will reflect that.
Look at Genesis 1:
1 In the beginning God created the heavens and the earth. 2 Now the earth was formless and empty, darkness was over the surface of the deep, and the Spirit of God was hovering over the waters.
26 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,[a] and over all the creatures that move along the ground.”
The “us” is believed to be the union of the Trinity.
Another key story where the Trinity is implied is in the sacrifice of Isaac. God the Father speaks to Abraham and instructs him to sacrifice the son he waited so long for.
From Genesis 22:
2 Then God said, “Take your son, your only son, whom you love—Isaac—and go to the region of Moriah. Sacrifice him there as a burnt offering on a mountain I will show you.”
3 Early the next morning Abraham got up and loaded his donkey. He took with him two of his servants and his son Isaac. When he had cut enough wood for the burnt offering, he set out for the place God had told him about. 4 On the third day Abraham looked up and saw the place in the distance. 5 He said to his servants, “Stay here with the donkey while I and the boy go over there. We will worship and then we will come back to you.”
6 Abraham took the wood for the burnt offering and placed it on his son Isaac, and he himself carried the fire and the knife. As the two of them went on together, 7 Isaac spoke up and said to his father Abraham, “Father?”
“Yes, my son?” Abraham replied.
“The fire and wood are here,” Isaac said, “but where is the lamb for the burnt offering?”
8 Abraham answered, “God himself will provide the lamb for the burnt offering, my son.” And the two of them went on together.
9 When they reached the place God had told him about, Abraham built an altar there and arranged the wood on it. He bound his son Isaac and laid him on the altar, on top of the wood. 10 Then he reached out his hand and took the knife to slay his son. 11 But the angel of the Lord called out to him from heaven, “Abraham! Abraham!”
“Here I am,” he replied.
12 “Do not lay a hand on the boy,” he said. “Do not do anything to him. Now I know that you fear God, because you have not withheld from me your son, your only son.”
13 Abraham looked up and there in a thicket he saw a ram caught by its horns. He went over and took the ram and sacrified it as a burnt offering instead of his son. 14 So Abraham called that place The Lord Will Provide. And to this day it is said, “On the mountain of the Lord it will be provided.
This event foreshadows the crucifixion of Jesus for our salvation. It is easy to see the Father and the Son represented in this story, but there are those who believe it was the Holy Spirit that enabled Abraham to be obedient to the Father. We believe that here in the OT because it is told to us many times in the NT that this is the work of the Holy Spirit to enable us to do the will of the Father.
A few years back I read through the Chronological Bible In One Year. I read it out loud with a friend every night over the phone. That experience helped me see the God’s story of/with mankind much more fully and I was amazed at how much of Jesus I saw in the OT. It was an experience that helped shape my understanding and the depth of my faith. It was like seeing a tapestry fully together rather than just individual threads. I recommend it. I do think the out loud part made a difference as well, as I was both reading and hearing, with two senses engaged.
Does any of this help?