seriously though, depending on the relationship, a threesome can work. its not for everyone but if the couple is sexually open and secure with each other, and looks at sex as something fun and something to be enjoyed, it could be fucking great.
since my wife and i have been discussing this topic too, i already know that i could never love anyone else but her, and if we do go for it, there is no doubt it would just be a fantasy and fun and my heart will always belong to my wife. and the other girl? the 2 candidates are close friends (one's an ex of mine and a close friend of my wife's now) and open, and they can separate sex form love too.
people swing and have other partners join in all the time, it can work, but make sure you're at that point first.