Both the OT and the NT tell us that Adam and Eve were the first two humans, and all humans descended from them; because of their fall into sin, all of us their children are plagued by that sinful curse.
I Corinthians 15:45 "And so it is written, The first man Adam"
Genesis 3:20 "And Adam called his wife's name Eve; because she was the mother of all living. "
Romans 5:12 "Wherefore, as by one man sin entered into the world, and death by sin; and so death passed upon all men"
Besides, Genesis 5:3 isn't the first reference to Adam and Eve having many children.
Genesis 1:27 "Then God said, “Let Us make man in Our image, according to Our likeness; let them have dominion over the fish of the sea, over the birds of the air, and over the cattle, over all the earth and over every creeping thing that creeps on the earth.” So God created man in His own image; in the image of God He created him; male and female He created them. Then God blessed them, and God said to them, “Be fruitful and multiply; fill the earth and subdue it; have dominion over the fish of the sea, over the birds of the air, and over every living thing that moves on the earth.”
God's admonition to Adam and Eve from the very beginning (not starting 130 years later with Seth), was to be fruitful and multiply and fill the earth.
Cain, Abel, and Seth were the only children of Adam named in the Genesis narrative, but by the existence of the people of the land of Nod living around the time of Abel's death, we can know without any doubt; and without calling the Biblical record into question; that Adam and Eve had many children between the time of Cain and Seth that simply were not detailed in Adam's geneology.