Adam and Eve had only three sons (Cain, Abel, and Seth), but no daughters: how did the world get populated?
The Bible actually indicates that Adam and Eve did, in fact, have daughters.
We also know that Seth was born after Cain had killed Abel:
So, when Adam was 130 years old after Cain killed Abel, Eve became pregnant and gave birth to Seth. So, did Adam and Eve have other children in between the time that Cain and Abel were born and Seth was born? We can't say absolutely, but we can't say absolutely not either. It certainly seems reasonable that other children would have been born during that time.
Genesis 5 does indicate that Adam and Eve had daughters as well (plural), though we are not told how many. The very minimum would be three sons and two daughters.
Apparently Jewish tradition holds that Adam and Eve actually had 33 sons and 23 daughters. This is not biblical, of course, but it aligns with the idea that Adam and Eve were probably extremely fruitful and multiplied greatly, as God had commanded them. The genetics were quite pure as well as the environment at that time. They lived a long time (Adam was 930 years old when he died according to Genesis 5), so they had a lot of time to have children.
Now, it should be noted that this doesn't completely answer the question of how the world got populated. The Bible teaches that the world was destroyed by a flood and that only Noah, his wife, his three sons and their wives survived.
So, the world has become populated today as descendants of those 8 people. Even Noah could possibly have had children after the flood as well as Shem, Ham, and Japheth.