Don't do it.

If you are prone to jealousy chances are you are not going to be able to curb it for a threesome, and even if you can it may come up after the fact. I would recommend not having one.

If all parties are not for it, then don't do it. It can damage and even end relationships.

Don't have one if either you and/or your girl/boyfriend would get jealous or angry after words. My girlfriend and I tried to have a 3 sum with another girl and it lasted about 3 minutes, because my girlfriend became heart broken becuase she thought I was going to fall in love with the other girl. We talked about it for a few weeks after and cleard things up and said we would never try it again. A little over a year later, and we started talking about her having sex with another woman and letting me watch. So soon after me, mygirlfriend, and the other girl(the same one as before) had 2 or 3 drinks together. We didn't get drunk and then we started talking about the 2 girls sleeping together and letting me watch and they agreed to do it. SO they did it in my and my girlfriends bed while I watched. 15 or 20 minutes later and I wanted in. So i started fucking my girlfriend and soon after i ended up fucking the other girl too and we had a 3 sum. The next day we talked about it, and our feeling have changed from the 1st one we tried to the last one we actually did do. We both enjoyed it and had fun. We ended up doing it 4 or 5 more times with the same girl. My point is, talk to all the parties involved. boy/girlfriend pluss the other guy/girl involved. Make sure no feeling or hearts will be broken, then slowly ease into it

Communicate. Lower you guard and voice all of your conscerns. Let your partner do the same. Establish boundries and draw a definitive line together as to what is acceptable and what isn't. If you both lay everything out on the table, and give it a go, you can always stop the action if things are getting a little out of hand. A threesome can easily destroy even a strong relationship. If there is any hesitation after you talk about everything, don't do it. Yes, it may be the sexiest thing you have ever done together, but it may also be the last thing you ever do together besides split up. If there are reservations, heed them. Your gut may be the best resource you can use.

If you think you'd be jealous, it's best not to do it. All three people need to be ready for the experience for it to be a wonderful thing. If you are still against it, maybe just roleplay it or talk dirty about it for a while and see if you could get past the jealousy. If you can't, that is perhaps the furthest it should go ... just fantasy.