I will show you why I believe the Trinity showed up in Genesis chapters 18 and 19.
Notice the angels and the Lord act like one, with the same authority as the Lord for they speak at the same time as the Lord.
Gen 18:1 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 (the Trinity) stood by him: and when he saw them, he ran to meet them from the tent door, and bowed himself toward the ground,
3 And said, My LORD (singular), if now I have found favor in thy sight, pass not away, I pray thee, from thy servant:
4 Let a little water, I pray you, be fetched, and wash your feet, and rest yourselves (plural) under the tree:
5 And I will fetch a morsel of bread, and comfort ye your hearts; after that ye shall pass on: for therefore are ye come to your servant. And THEY SAID, So do, as thou hast said.
6 And Abraham hastened into the tent unto Sarah, and said, Make ready quickly three measures of fine meal, knead it, and make cakes upon the hearth.
7 And Abraham ran unto the herd, and fetched a calf tender and good, and gave it unto a young man; and he hasted to dress it.
8 And he took butter, and milk, and the calf which he had dressed, and set it before them; and he stood by them under the tree, and they did eat.
9 And THEY SAID unto him, Where is Sarah thy wife? And he said, Behold, in the tent.
10 And he said, I will certainly return unto thee according to the time of life; and, lo, Sarah thy wife shall have a son. And Sarah heard it in the tent door, which was behind him.
11 Now Abraham and Sarah were old and well stricken in age; and it ceased to be with Sarah after the manner of women.
12 Therefore Sarah laughed within herself, saying, After I am waxed old shall I have pleasure, my lord being old also?
13 And the LORD said unto Abraham, Wherefore did Sarah laugh, saying, Shall I of a surety bear a child, which am old?
14 Is any thing too hard for the LORD? At the time appointed I will return unto thee, according to the time of life, and Sarah shall have a son.
15 Then Sarah denied, saying, I laughed not; for she was afraid. And he said, Nay; but thou didst laugh.
16 And THE MEN ROSE UP from thence, and looked toward Sodom: and Abraham went WITH THEM to bring them on the way.
Shalom, it continues ….