The appearance of Jesus pre-incarnation in human flesh is called a Christophany.

While it is never stated that Jesus actually appeared to people in the Old Testament, scholars point to a few incidents where they believe it’s possible.

Then Melchizedek king of Salem brought out bread and wine. He was priest of God Most High, and he blessed Abram, saying, ”Blessed be Abram by God Most High, Creator of heaven and earth.
And praise be to God Most High, who delivered your enemies into your hand.” Then Abram gave him a tenth of everything. Genesis 14:18-20

Hebrews 5-7 in the New Testament talks about Melchizedek and draws clear lines between him and Jesus, saying:  

• King of Salem means King of Peace or Prince of Peace, a name given to Jesus.
• He was a priest of God. Since God seems to working exclusively through Abraham in defining his nation, where did Melchizedek get his inspiration, not being born of the lineage of the Levitical priests?
• He blesses Abraham.
• Abraham tithes to the priest, an offering previously given only to God.
• Melchizedek also brings out bread and wine, alluding to Jesus’ last supper.

The LORD appeared to Abraham near the great trees of Mamre while he was sitting at the entrance to his tent in the heat of the day. Abraham looked up and saw three men standing nearby. When he saw them, he hurried from the entrance of his tent to meet them and bowed low to the ground. Genesis 18:1-2

Abraham is visited by three “men” four chapters later. They could be all angels or all three persons of the Trinity. More likely it was Jesus and two angels. Abraham called one of the “Lord.” One of the men spoke with authority, prophesying Sarah’s pregnancy and the destruction of Sodom and Gomorrah. Genesis 18:26 uses the term “The Lord says.” Then one of them sends the other two to Sodom and Gomorrah, showing a command hierarchy.

Some want to ascribe every time the term “Angel of the Lord” is used to an actual appearance by Christ. That would make Jesus a lot more active in the Old Testament. '

• In the Garden of Eden giving animal skins to Adam and Eve
• Meeting Hagar in the wilderness
• Wrestling Jacob by the fire
• With Moses in the burning bush

It’s hard to say which times Jesus appeared and which he sent an angel, but it’s likely some of them were the pre-incarnate Christ.