There’s really no knowing before hand. Do you want this to be an ongoing thing? One time? Do you have ground rules? Are you going to keep it about sex, or are you going to try to include emotions? What is the existing relationship of the three?
Sometimes the man thinks it’s cool (which is to say, hot) if his girl and another are getting it on. Some men like to have both women attending to them.
It’s best to talk it through before hand so you know you’re all on the same page. Be as detailed as you can. Continue talking as you do it. If someone is uncomfortable, talk about it. Try not to hold in resentments for later. It’s hard, but if you can do it, it’s better.
I will tell you this. My wife is not one of the women I ever had a threesome with. I’m pretty sure that those extra relationships played some role in the breakups.