Threesome with my fwb and another girl?

So I'm in a dilemma 😂 I've been friends with benefits with this guy for over a year, I've only slept with him but I'm attracted to girls too and always wanted to do something with a girl. He asked me about a threesome with another girl, at first I didn't know what to say cause I didn't think I'd be very self confident to have a threesome and he said it would be her first time doing something with a girl and that relieved me a bit. She's been tested and all her tests came back negative, so I'm like well why not? But still don't know, any help? 😂