The God of Abraham, Isaac, and Jacob is the Lord Jesus Christ before His incarnation as the God men had seen in the Old Testament.
John 8:56
Your father Abraham rejoiced to see my day: and he saw it, and was glad. 57 Then said the Jews unto him, Thou art not yet fifty years old, and hast thou seen Abraham? 58 Jesus said unto them,
Verily, verily, I say unto you, Before Abraham was, I am. 59
Then took they up stones to cast at him: but Jesus hid himself, and went out of the temple, going through the midst of them, and so passed by.
Genesis 18:1And 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, 3 And said, My Lord, if now I have found favour 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 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 fetcht 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.
That was the day Abraham was glad because He told him that he would have a son.
God appeared and was seen by Isaac & Jacob too.
Genesis 26:1And there was a famine in the land, beside the first famine that was in the days of Abraham. And
Isaac went unto Abimelech king of the Philistines unto Gerar. 2
And the Lord appeared unto him, and said, Go not down into Egypt; dwell in the land which I shall tell thee of:
Genesis 32:24
And Jacob was left alone; and there wrestled a man with him until the breaking of the day. 25 And when he saw that he prevailed not against him, he touched the hollow of his thigh; and the hollow of Jacob's thigh was out of joint, as he wrestled with him. 26 And he said, Let me go, for the day breaketh. And he said, I will not let thee go, except thou bless me. 27 And he said unto him, What is thy name? And he said, Jacob. 28 And he said, Thy name shall be called no more Jacob, but Israel: for as a prince hast thou power with God and with men, and hast prevailed. 29 And Jacob asked him, and said, Tell me, I pray thee, thy name. And he said, Wherefore is it that thou dost ask after my name?
And he blessed him there. 30 And Jacob called the name of the place Peniel: for I have seen God face to face, and my life is preserved.
How do we know it was not God the Father men had seen rather than the Son, as Jesus did say Abraham had seen Him in his day?
John 1:18No man hath seen God at any time, the only begotten Son, which is in the bosom of the Father, he hath declared him.
John 6:46 Not that any man hath seen the Father, save he which is of God, he hath seen the Father.
Jesus testified that He has seen the Father as He is of God.
@Keiw1