So, a couple of questions -
Is this primarily about the sex? Most of what you have posted seems to talk about sexual experiences, and very little about emotional and loving relationships.
At the time that your wife talked about the threesome, thinking back, was it presented as a "hot fantasy" during love-making, or was it something that she talked about outside of that context?
Bottom line is - the two of you made an agreement to be monogamous. her "and the two of them lived happily ever after" is not crazy at all, in my opinion - it's a natural consequence of the promises and commitments that the two of you made to each other. You are essentially asking her to change that.
If your wife doesn't want to do this, doesn't want to change the contract that the two of you have, then you need to respect her views, and not try to cajole her into changing her mind. Or you need to end the contract/marriage. It seems like the respectful thing to do, to me.
"Listen, or your tongue will make you deaf." - Native American Proverb