Let's have a bisexual threesome

One day, when I and my wife was in a party where were lots of hot couples and singles. I found some of them are swing relationship. After having some drinks, my wife said:"look at that woman, she is keeping staring at us." She was a beautiful woman in tight dress which shows her perfect body. And she came for us, said:"Hi. Let's have a threesome." What she said was surprising us, and she said she was a bisexual, my wife and I were exactly her type.