I've been with my girlfriend for a decade now and about three years ago we decided to go ahead and experiment a bit. I got to see her with another man (something I really wanted to experience) which is something we had joked about and flirted with the idea before that, but we were too nervous to take the plunge. Then one day the perfect situation came up, we talked about it and we went ahead and gave it a go. It was fucking awesome and we've done it several times since then, but at first we really were worried about potential negative ramifications.
The thing is, I never pushed it on her. We'd talk about it and I think she liked aspects of the idea, but it sounded too weird to her at first. So it was one of those things that I never expected to happen or tried to make happen, but I made sure that she knew I'd be up for it if she ever changed her mind. And then one day she did and now it's an occasional extra thing we do from time to time. Not only that, but I think it freed us up about other aspects of sexuality as well. We've definitely become open to trying new things.
There's nothing wrong with wanting to try a threesome, but it really does have to be something she wants as well. If you made it clear that you're up for it and she knows that it would never cause you to be jealous or looked at her in a negative way, then just be cool and see what the future brings. If she's flirting with friends and letting you know that she still has sexual attractions to others, maybe one day she'll surprise you and decide to experiment with you and another. Just let her know that you won't be disappointed if it doesn't happen. Pressuring her will just make her feel bitter towards the whole idea.
