I was 6 years into a relationship and was very interested to try a threesome, but I was very nervous about how my partner would take the suggestion. We would talk about hot guys a lot, sometimes watch porn together, etc., but for some reason making the suggestion seemed too frightening. Finally at one point a guy we knew expressed interest in doing a threesome with us, so that forced the question. We finally talked about it and realized we were both extremely interested and yet had both been too afraid to even raise the subject.
We decided that we needed some rules about it to make sure that our fears were addressed: it always had to be a guy who was equally interested in both of us (nobody feels left out); we had to always make sure both of us were getting attention, including nudging the other to take over or directing the third to give attention to the other (again, nobody feels left out); and we always made it clear to the third that since we were "new" at this and nervous about it, if at any time one of us felt uncomfortable, either of us could push the "eject" button and everything stopped... no excuses required, no hard feelings, no questions asked.
Those rules worked great, only once did we ever have an "eject" situation and it went smoothly. My partner hit eject, just said he wasn't "feeling it," so we stopped, cleaned up, and I drove the dude home. Because we'd discussed this before, he was disappointed but not upset and was understanding. Afterwards, my partner and I talked about it and figured out what was wrong about that particular situation and all was well -- we even met that guy again a few months later!
At some point, about a year into some pretty hot threesomes, my partner said to me one evening: "I keep thinking about how much hot sex we missed over the first 6 years because we were too afraid to talk about it!" At that point I realized that we were not just partners, we were "partners in crime"... not literal crime, but in term of being both down for mischief and scandalous behavior. We weren't afraid what the other thought... we knew we could count on the other to be right there along for the ride.
To me, that's the lesson: talk about it! You might be surprised by the response. Phrase it as curiosity, not a demand or a requirement. "I saw some porn and the threesome looked very hot. Have you ever had a threesome? If we found the right guy, is that something you'd consider?" Phrasing like that helps the response be something like, "yeah, with the right guy..." which is a great way to express interest without making some kind of deadline or making it about some person in particular.
Phrasing it generally, conceptually, makes it easier to say yes to the concept without committing to a specific act at a specific time. Never make a deadline or hard requirement... it always has to be when the feeling is right for the first time. That's the best way to approach it. If there's hesitance, it's easier to say you're "open to it if the opportunity comes along, but not actively looking for it..." and then let you both get used to that idea without there being any pressure. I'm willing to bet that after a while, the daydreaming about the possibility will help encourage it to happen.
Also make it clear that if you wanted to try it it, you'd establish rules (maybe no fucking, no sleeping over, no kissing... whatever makes you feel more comfortable and non-threatened). Make it clear that anyone can press "eject" for any reason, or no reason, and there's no hard feelings. Go slow, make sure you keep communicating. And I bet that you'll eventually look back and ask yourself why you were so afraid as you lay there with some third dude bathed in you and your partner's cum.
Good luck!