A threeway relationship can work, but it would take a lot of effort and cooperation and really some luck as it would be difficult to manage time with three people. I mean its difficult for my friends and I to schedule outings, but I guess being in a relationship one is more determined to make outings work.
Anyway, as far as being a participant myself, it would have to be a special circumstance, and it would have to be a guy, guy, girl pairing and I think that's a bit selfish of me really.. lol. You also have to separate fantasy from relationships and realize that even though you may like two people and they may be good friends, the reality of the relationship may not be strong... even though one wants it to be (fantasy). And I agree with those who have said that it should be a for real threesome and not an open relationship. Person A, B, and C should all do everything together.. I mean in a two way relationship we don't go on dates by ourselves... so why leave you one person and go on a date with the other? Things like that would drive a wedge between the group eventually, even if there is a lot of trust involved. So all the variables in making a successful relationship is doubled and personally I've not been really good at managing a successful relationship with one person.
Later in life I think I'd be more prepared for something like that, when I'm more mature and settled within my own life. Right now I'm busy trying to figure out what I want to do, where I want to settle down and live, and really I wouldn't want to burden myself.