Contradiction with Exodus 33:20
In Genesis 18:3, God is perceived in a physical form during Abraham's encounter, whereas Exodus 33:20 states that no man can see God's face and live.
Exodus 33:20: And he said, Thou canst not see my face: for there shall no man see me, and live.
Contradiction with John 1:18
Genesis 18:3 involves a tangible encounter with God, but John 1:18 supports the idea that no one has seen God at any time.
John 1:18: No man hath seen God at any time; the only begotten Son, which is in the bosom of the Father, he hath declared [him].
Contradiction with 1 Timothy 6:16
Genesis 18:3 suggests direct contact with the divine, yet 1 Timothy 6:16 describes God as dwelling in unapproachable light, whom no human has seen or can see.
1 Timothy 6:16: Who only hath immortality, dwelling in the light which no man can approach unto; whom no man hath seen, nor can see: to whom [be] honour and power everlasting. Amen.