In Genesis the most often verses used to try to "prove" the trinity are the "Let US make man in OUR image" verse and the story of the 3 angels of "the LORD" that visit Abraham. Both of these are an attempt to say Jesus was there with God from the very beginning and interacting with the fathers and prophets. But we can see from Hebrews 1 that this isn't the case at all.
Hebrews 1:1 God, who at sundry times and in divers manners spake in time past unto the fathers by the prophets, 2 Hath in these last days spoken unto us by His son, whom He hath appointed heir of all things, by whom also He made the ages (aiōnas)
Hebrews specifically tells us that God was not speaking through His son Jesus until "these last days". When God spoke to the fathers (in the time of the Old Testament) He used prophets to speak to them.
Genesis 1:26: 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.
The bible interprets itself, to see how God made people in His image (singular) but also made us in "our image" (plural) click here: Genesis 1:26.
Genesis 18:1-3: And the LORD appeared unto him in the plains of Mamre: and he sat in the tent door in the heat of the day; 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, and said, My Lord, if now I have found favour in thy sight, pass not away, I pray thee, from thy servant:
Genesis 18:22: And the men turned their faces from thence, and went toward Sodom: but Abraham stood yet before the LORD.
Genesis 19:24: Then the LORD rained upon Sodom and upon Gomorrah brimstone and fire from the LORD out of heaven;
Some trinitarians will use these verses to say that since these are "men" and Abraham bowed down and called them "My Lord" that they must be the trinity, or at least one of the men must be Jesus, because Abraham was still standing "before the LORD" while "the LORD" rained fire out of heaven. To see what was really going on, click here: Genesis 18-19.