1 Now these are the generations of Esau, who is Edom. 2 Esau took his wives of the daughters of Canaan; Adah the daughter of Elon the Hittite, and Aholibamah the daughter of Anah the daughter of Zibeon the Hivite; 3 And Bashemath Ishmael’s daughter, sister of Nebajoth. 4 And Adah bare to Esau Eliphaz; and Bashemath bare Reuel; 5 And Aholibamah bare Jeush, and Jaalam, and Korah: these are the sons of Esau, which were born unto him in the land of Canaan.
The narrative is now paused for a chapter as the Bible recounts the genealogy that proceeded from Esau. We are reminded that Esau had two wives of the Hittite and Hivite nations, and a third from the daughters of Ishmael.
As discussed earlier, Esau only married the third wife to appease his parents, who wished him to keep his family within the covenant lineage. However, I am not sure that marrying the daughter of Ishmael fit that bill. Yes, the covenant people would emerge from the children of Abraham, but not all of the children of Abraham would be part of that group. Isaac would be part of the covenant and not Ishmael, just as how Jacob was now part of the covenant but not Esau.
In any case, Esau only had one son from two of the wives, but three from the third. Five sons were enough to assume his lineage would be carried forward, though, and the following verses will show that it was indeed.