Seth was the third son of Adam and Eve. He was born after Cain killed Abel and God sent Cain away (Genesis 4:25–26).
Adam was the son of God because he had no earthly father. God created him. When Satan tempted Jesus in the wilderness, Satan repeatedly says to Jesus, “If you are the Son of God...” Adam was the first son of God and failed when tested. But Jesus, the Son of God in the fullest sense, successfully resisted temptation.