Thank you for your reply. To clear it, let's see this :
Genesis 4 : 13-15
"This punishment is too hard!" Cain said. "You are making me leave my
home and live far from You. I'll have to wander about without a home,
and just anyone could kill me."
"No!" the Lord answered. "Anyone who kills you will be punished seven
times worse than I am punishing you." So the Lord put a mark on Cain to
warn everyone not to kill him.
Taken from Contemporary English Version
It is said that there is someone else beside Adam,Eve, and Cain,
because Cain was afraid he would be killed if he left his home. So, who
is "everyone"?