If my wife said she wanted to try a threesome I'd be happy to, with due planning and agreed procedures for bscking out if we didn't like the way it went. I wouldn't suggest it myself, psrtly because it's not something I crave, and also because we both flirt quite heavily with others and we know how far to go with that so as not to upset each other.
I'd also bear in mind that, while not an unbreakable rule, men are generally programmed to want multiple partners more than women do.
Sometimes the fantasy is more useful than the fact of it - if you can talk about others you might have fancied or flirted with while enjoying sex together, there's no reason why it couldn't spice up the session. Obviously you'll have to do what works for you as a couple. If you're single, then it's a whole different ball game although even at my most active I never sought out threesomes. I quite like the idea of two women ganging up on me but they'd have to just do it and it's never happened yet!