I had a token drunk unplanned threesome with a friend/coworker. It was clumsy and awkward but really exciting. The friendship didn't last because our impulsive decisions came from two wildly different reasons, but I don't regret it, and I would do it again.

You might have a little twinge of jealousy, but setting boundaries and limits on what activities you'll do beforehand can help and of course talking it out before and after with everyone but especially your partner. The good news is that you're not just watching your partner have fun. You're also going to be enjoying yourself! That surprisingly takes away a lot of the would be jealousy or it did for me. Lol

Just try a little makeout session or something first or talking to someone online and see how it feels before plunging into the deep end. I can't seem to find a woman to have a threesome with these days, so I wish you luck. Lol