How does one get his wife to entertain the idea of a threesome with another woman?

Answer

I agree, start with fantasy roleplaying and see how she feels about it. You'd be shocked with how many people that I know that want to be with another woman but are afraid to tell their significant other. If she is up for it there, SLOWLY try suggesting it for real. If she's not interested, be thankful that she did it during roleplay and let it go.

Answer

She has to have bisexual or bicurious tendencies to begin with. If she is curious in being with another woman, convey the idea to her in the form of a fantasy. Roll play the fantasy and then begin to ask for names of women who she'd be interested in being with. Encourage her to explore that side of her sexual appetite and reassure her that you would have no problem with her having sex with another female. If this happens, you have a great opportunity for the threesome. As she has now explored and realized her fantasy, then it is time for you to explore yours, and she will most likely be compliant.

Word of warning. If you do get to the threesome stage, don't pay more attention to the other woman.

Answer

My gosh! Whatever happened to the vows you took when you got married???? I don't believe anywhere in those vows it said, "my wife follows me in any direction I wish to go and she's got a smile on her face doing it!" Marriage is about honesty, integrity, UNITY, and loyalty, and it doesn't appear you want any of these things. Instead of confronting her with this question perhaps you need to think about yourself and where YOU are heading and perhaps the single life is more your style.