I have never had a 3some involving my spouse.
I have to rain on your parade, but I think it is better as a fantasy than as a reality.
But then again, what do I know? I've never had one with a spouse.

My issue is picking the person. I don't want someone who wants to intrude on my life. We are monogamous and want to be with each other only. We aren't into having someone else move in and we aren't into having someone who wants to hang around forever. I want someone who, basically, I can use for this fantasy and send them on their way. Well, you just never know who you are going to get. You could get someone who is totally cool, but you could get someone who falls held over heels for one of you or both and starts stalking you. You also need to be careful in who you pick because you need to know somethings about them like if they have diseases, criminal record, or they are on birth control (if female, because the last thing you'd want is to pay out child support over an accident.) You have to put trust in this person, your partner, and yourself. It could hurt your relationship, but it could improve it too.

Bottom line is you need to decide if it's something for your relationship and decide together what kind of person you are looking for.

My boyfriend and I have had this conversation a few times over the past year or so.
My most wanted fantasy is to be DP'ed. His is to have two women (He says he'd like either two of me or me & Rihanna, lol) fawning over him.
I get a little jealous when I think of other women touching my boyfriend... and thinking about his penis being inside them just ticks me off.
He says he gets mad just thinking about another guy inside me, but, since he's the only guy I've had sex with - he can understand why it's a fantasy of mine and why I would want to experience another man.

I think it really depends on how comfortable you are with each other as a couple and how trusting you are.

However, I would never add a third person to the relationship. To the sex, maybe. & For me, it would have to be a completely random person (who wasn't a serial killer) that I would never see again.