Is it wrong to have a 3sum with my gf?

Im a female and my girlfriend and I have been together for almost 7months and we have a Great sex life. We both have experienced orgasms and we have sex like 5 days a week and usually more than one time a day (if you wanna get exact). Well anyway, I always joked about a 3sum involving ne, her and another girl. But the girl has to be a 10. She always laughed about it and said she's down with it but i never know if shes being honest or just playing. I want to have a 3sum because neither of us have done it before and it would be a cool experience. But i have really strongh feelings for my girlfriend and she feels the same with me so i feel that we will get jealous when wee see the girl on either one of us. Alo if the girl the attracted to one of us more than the other than the other will get jealous. Its complicated i iknow, but i dont want this to mess up our relationship later on. We're both freaky as hell and have an exotic sex life so idk, this would be cool but i have doubts.. What should i do?