Or HaChaim on Genesis 5:3/אור החיים על בראשית ה׳:ג׳

1 ויולד בדמותו כצלמו. He reproduced in his likeness and in his form. The Torah stressed this positive development, seeing that neither Cain nor Abel represented ideal offspring, both of them dying either almost immediately or their offspring during the deluge. During the 130 years between the birth of Cain and Abel and that of Sheth most of the pollutant Eve absorbed from the serpent had been expelled making it easier for a promising human being to emerge from her womb. We have a tradition that the third son is the most saintly, a tradition that proved itself with both Levi and Moses. It probably originated with the birth of Sheth.