They had more than three sons. Cain, Abel, and Seth are mentioned by name, but they had other sons and daughters.
Adam's Descendants to Noah
GENESIS 5
This is the book of the generations of Adam. When God created man, he made him in the likeness of God. 2 Male and female he created them, and he blessed them and named them Man when they were created. 3 When Adam had lived 130 years, he fathered a son in his own likeness, after his image, and named him Seth. 4 The days of Adam after he fathered Seth were 800 years; and he had other sons and daughters. 5 Thus all the days that Adam lived were 930 years, and he died.