My sister is in a wonderful and loving equal triadic relationship with another woman and a man. They all live together, sleep together, share a bed and bedroom. My sister and her boyfriend have been together for several years. Their girlfriend is a more recent addition, a little less than a year. They are all 3 very happy and it works very well for them. I don't, however, think this is a good situation for everyone. I'm honestly not sure I'd be able to be in a triad or non-monogamous relationship. If I did, it would be only other women, no men, since I'm not interested in men. I think it takes a great deal of trust and communication and honesty. These are, of course, vital to any good relationship but I think that becomes even more true with more people or some non-traditional type of relationship. I don't think it's for everyone but for my sister and her partners, thus far I think it's been an amazing thing for all of them.
I've known many polyamorous individuals and couples and for some it works perfectly while for others it causes nothing but strife and problems. I think it's something you truly have to decide for yourself and your relationship on an individual basis. There's no rules to this game except the ones you decide on with your partner(s) and no right or wrong thing to do except what's right or wrong for you and your relationship.
If it's a one time try it out kind of thing, a long term friend may not always be the best choice since there may be feelings afterward that could damage or destroy a friendship. Personally, I'm not one for anonymous one-night-stand-type sexual encounters but some people are and that's fine. For more serious relationships, friendship is a vital part of any good relationship, especially lasting ones, but I think there's many good reasons why a lot of people choose not to become involved in a romantic or sexual relationship with a good friend. I think you should look at it from the same perspective you would if you were single and interested in pursuing a relationship with a friend, except maybe examine even more carefully since there are extra hearts and minds on the line. If considering embarking on a sexual or romantic relationship with a friend, I would ask myself if the potential rewards of such a venture would outweigh the risks of loosing that person as a friend and part of my life if things go badly. This is ALWAYS a possibility, no matter how much you promise each other and yourselves that nothing will ruin your friendship. There is ALWAYS something that can destroy even the very best of friendships. Always. For me, personally, a one time threesome experience would not be worth the risk. A long term triadic relationship might be if I thought it really had a shot and I really cared about both the people involved and they for me and for each other and we were all on the same page about what we wanted. But in any case, it would never be something I'd recommend rushing into or engaging in carelessly. And, bear in mind the potential damage adding a third party to a relationship can have on the existing dyad as well and weigh those risks against the possible benefits. I'm not saying it's a bad idea to have a threesome. Many, many couples do it successfully and enjoy the experience and my sister and Velvet Dragon are both prime examples of threesomes working out tremendously well as more serious relationships. It's just something that I think should be considered VERY carefully and cautiously with lots, and I mean LOOOOOOOTS, of communication, trust and honesty.