Opinions on a 3 some

My husband's biggest fantasy is to bring another woman into the bedroom, although I can see it being fun, it gives me so much anxiety to think about. I want to hear from both sides, the significant other, and the 3rd wheel, how did you feel during this process, how did you set everything up (or was it a spur of the moment) how do you find a female interested in that but doesn't want to meddle in your relationship. Did it affect your relationship negatively or positively. I find myself comparing myself to women, I don't want it to be with somebody I consider better looking than me, but I don't want to be grossed out. I had one before in a past relationship but it was mostly just watching my guy do another girl, he didn't listen to any rules I set out, I left in the middle because i felt left out. How do I stop that from happening? Advise?