First Time For Girlfriend

My girlfriend (21) is pretty open about sex and we have a lot of fun in bed. Friday night I watched her suck our friend's dick and swallow his cum. It was the first time she had done something sexual with a guy other than me. As I watched, I got a very serious erection. As a joke, I had told her she should blow our friend. I really did not think she actually would. But she undid his pants and took his dick out and started sucking. Neither my friend or I tried to stop her. Long story short, it opened a new chapter in our sex life. The three of us are going camping next weekend and our plan is for my girlfriend to sleep with both of us for a couple of days. I am really turned on by knowing I will see her fuck our friend. The anticipation is killing me. She is also excited by the idea of a two day threesome. It obviously will be her first time to fuck another guy or to experience two guys.