How do I(f) ask my bestfriend(f) to have a threesome without ruining the friendship or being weird?

We have actually been best friends for over 10 years now and we have never done anything sexual together. When we go out and have drinks we do kiss each other a lot and in the past we have kissed each other’s boyfriends all together but it hasn’t escalated from that. We recently went out together and kissed here and there but we got in a photo booth and took pictures rubbing our boobs together. Well that kind of sparked something. I showed my boyfriend the pictures because I knew he would love it and since then we have been hot and heavy talking about my boyfriend watching us fuck. The more I think about it the more I want to do it but I’m so scared to ask her if she would be down for it. On one hand I think she would but on the other she might be like ew you weirdo 🤣
She also has a boyfriend but I really don’t want him there lol I feel like he would ruin the vibe so I would have to ask about that too.
I want my boyfriend to do more than watch so that would need to be consider as well. But we are both very attractive women so if she wasn’t ok with him being involved I feel like it would be just as hot if he was there watching us fuck.
SOS guys, I’m scared to ask her but I can’t stop thinking about fucking her