I'm in a serious relationship, and we've had a couple threesomes. I think we would both like to do them more, and we've talked about it a few times; we just haven't actually gotten around to doing it. I don't have many male friends who I would both trust around my gf and feel comfortable in a threesome environment; conversely, my gf doesn't have many attractive female friends nearby. Though we've been together for a long time, we're still pretty iffy about flirting too much with the opposite sex, so that's a big reason we haven't had any new threesomes in awhile.
That said, I think that if we could just get over ourselves, threesomes would enhance our relationship rather than hurt it. The ones we had were very fun, and we all enjoyed ourselves; we also didn't have any problems after the fact. I think they're great for adding variety to a relationship-- I don't mean to say, by any stretch, that I don't love my girlfriend; however, I've been with her for a long time now, and considering that I don't cheat on her, the opportunity to "legit-ly" add another girl to the mix interests me. And I'm sure that, though she probably wouldn't admit it to me, she'd be pretty excited to have someone new in the bed from time to time.
I would say, though, that if you're considering doing it while in a serious relationship, you should definitely have a serious conversation with your significant other and really get everything out on the table. If one person isn't really into it, and is only doing it because their significant other seems really excited, it can cause problems. And both of you need to be really comfortable with each other, and that you'll continue to be together after opening up your relationship.
Best of luck..