My friend asked me if I want a threesome...?

My girl friend who I've known for years asked me in college today for a threesome. We're both 17, and she said she wants to ask someone else but he's a boy. I know him, but I'm not too sure now.

I speak to this guy every day but basically having sex with the same girl at the same time? I'm not sure

I've suggested to her maybe ask one of her friends who's a girl, but she didn't really want to.

What should I do? I'm sexually active anyway so please don't give me "oh you're too young for this" - Yes, I am but I won't heed your advice; I've been having sex for a few years now.

So has anyone else had a threesome? If you're male, have you had sex with another girl and another lad? If you're female, have you had sex with another girl, and one lad?