The bible doesn't say there were only the 3 people at the time. In fact, Cain took his wife with him when he left the region Adam and Eve lived in. We don't know how long of time there was between his birth and Seth's birth, which happened around the time Cain left, but there could have been some 100 children of Adam and Eve around by then, and maybe some grandchildren.It is written, "For precept must be upon precept, precept upon precept; line upon line, line upon line; here a little, and there a little: (Isaiah 28:10ff)";
The long held belief that GOD ONLY used 2 originally sinless people to begin populating the entire earth, DOES NOT stand up to "sound" doctrinal scrutiny when soundly, carefully and reasonably examined (for example, who are those that Cain is worried about, after murdering his brother Abel, when he says, ". . . I shall be a fugitive and a vagabond in the earth; and it shall come to pass, that every one that findeth me shall slay me" in Genesis 4:14, if there is ONLY Adam, Eve, & Cain populating the earth at that time???); and is shown to be very doubtful &/or questionable as can be, especially when one considers, AMONG SO MANY OTHER infallible proofs, how HE (GOD) seems to have had to use 6 SIN TAINTED people (Noah's 3 sons & their 3 wives) to begin RE-populating the entire earth after the Flood.
It really doesn't help your argument when you have to tell me how sound it is in comparison to mine. It is pure bluster, along with logical fallacy of assuming the conclusion ("begging the question").
What you have to show is that there is no possibility from the scriptures that there were any other people descended from Adam and Eve on the earth at that time. And you can't. In fact, Paul talks about all mankind being descended from Adam: [Act 17:26 KJV] 26 And hath made of one blood all nations of men for to dwell on all the face of the earth, and hath determined the times before appointed, and the bounds of their habitation;
and Moses says Eve was the mother of all the living: [Gen 3:20 KJV] 20 And Adam called his wife's name Eve; because she was the mother of all living.
So Cain's wife was either his sister or his niece, perhaps, descended from Adam and Eve. Those that might slay him were brothers or sisters of his and Abel's. You can see why they might slay him, if they were brothers or sisters of Abel.