I am bisexual and I have found more often than not when I am dating a man he will eventually suggest a threesome. I have had group sex before. It was a foursome though, not a threesome, and I found that it wasn't for me. Sex is a very emotional thing for me and I need to feel that emotional bond between myself and my sex partner before engaging in sex in order to enjoy it. I also feel a little jealous and/or rejected when the focus is on another woman...unless of course it's just myself and another woman
It's really a personal choice. I have heard of relationships really benefitting from inviting someone new into their bed, while others have been destroyed by it. Some just try it and find it's not for them. Make sure your relationship is strong and you both discuss it thoroughly before hand. It's important to know how you both feel about all possible outcomes...like if 1 enjoys the experience and the other does not. make sure that wouldn't harm your relationship in anyway.
Anywho..I'm babbling...hope I helped.