In Genesis 17, with the Covenant of Circumcision, God tells Abraham that Sarah will have a son. Abraham was very surprised and ‘”laughed to himself.” He must have told Sarah what the Lord told him. After all, you need a woman to have a child with. But then in Genesis 18, we find the story of the Three Visitors: ‘Then the LORD said, ‘I will surely return to you about this time next year, and Sarah your wife will have a son.’ Sarah reacts as if she is hearing this prediction for the first time.
Both these stories cannot be true at the same time. If God had already told Abraham at the Covenant of Circumcision that he would have Isaac, what was the point in sending three angels to make the same prediction again?
Most likely, Isaac was not the son in Covenant of Circumcision, but after the Babylonian Captivity when Torah was rewritten from memory, Jewish scribes must have added few verses (Genesis 17:15-22) to shift the focus on Isaac and made him the protagonist. If you skip these verses and jump from Genesis 17:14 to Genesis 17:23, there is perfect continuity in the text, otherwise there isn’t.