John Gill Commentary Genesis 49:31

"there they buried Abraham and Sarah his wife; there they buried Isaac and Rebekah his wife; and there I buried Leah:" — Genesis 49:31 (ASV)

There they buried Abraham and Sarah his wife
Abraham buried Sarah there himself, and his two sons, Isaac and Ishmael, buried him there:

there they buried Isaac and Rebekah his wife
we have no other account of the death of Rebekah, and her burial, but here; it is probable she died before Isaac, and that Isaac buried her in this cave; and here Esau and Jacob buried him:

and there I buried Leah
of whose death and burial we also read nowhere else but here; it is probable she died before Isaac, and that Isaac buried her in this cave; and here Esau and Jacob buried him: