The question assumes a literal young-earth, literal creation and Adam and Eve as the literal first and only people interpretation of Scripture, so this answer comes from that view.
The common response from pretty much every Apologetic YEC organization is that they married their sisters. There were no other women in the world. And besides that, there were no laws forbidding marriage between brothers and sisters, but had there been; only God was in authority at the time, and he already knew.
As explained on ChristianAnswers.Net
...Even though only these three males are mentioned by name, Adam and Eve
had other children. In Genesis 5:4 a statement sums up the life of
Adam and Eve—“And the days of Adam after he had fathered Seth were
eight hundred years. And he fathered sons and daughters.” This does
not say when they were born. Many could have been born in the 130
years (Genesis 5:3) before Seth was born....
Also from biblestudy.org
ANSWER: The debate over who became the wife of man's first two
children has gone on for centuries. The short answer is that the
brothers had to marry their sisters. Man was, at first, only two
people, Adam and Eve. The couple's first three children were Cain,
Abel and Seth: "After he begot Seth, were eight hundred years; and he
had sons and daughters" (Genesis 5:4).
How many sisters were potentially available for Cain and Abel to
marry? The Bible does not reveal exactly how many children Adam and
Eve produced. It is safe to assume, however, that since Adam lived 930
years, he had PLENTY of time to take full advantage of God's command
to procreate and fill the earth!
And the answer to the prohibition against Incest is consistent among apologetic groups.
For future research, I recommend a Bing or Google search on "Who did Cain marry". You'll find that the answers that you find from those who hold the literal interpretation of Genesis, the answer is consistent.