Two is company, three is extra

There were two extra pairs of gigantic teenaged boy shoes at the backdoor when I got home.  As I approached the dining room where I expected to find Elder Son and two friends tapping away at computer war games, I talked to myself outloud.  "I sense two extra boys in my house!"
When I rounded the corner, sure enough, three boys occupied the table with their faces buried in screens.
"There's three of us!" one boy corrected me.
"Three extra boys!" another agreed.
I pointed to Elder Son and said, "He doesn't count as extra.  He comes standard."