I Want to Convince My Wife

I have fantasized about having threesomehookups with my wife and a third woman for many times and I want to convince her to do it. When I mentioned it to her for the first time, my wife said no directly and told me that there is no room for negotiation. But I don't want to give it up so easily since living a swing lifestyle is always my dream. So, I want to find a way with which I can get my wife in a threesome, without any cheat, of course. Maybe one day I will succeed and then I will be the happiest man in the world.