It is totally different thing to fantasize and make fantasy real. However, these days many couple start indulging in threesome and enjoying the forbidden act.
It might be possible that your marriage may get distressed afterwards because of jealousy and emotional issue in threesome. If you are ready to face all the consequences of threesome then you may think of going ahead.
Discuss your idea with your wife for threesome in real life and see her reactions. If she is positive then there is nothing to worry except you need to find compatible partner, who wants to share bed with you both people.
If she does not like the idea of threesome in real life then without second thought you should stop this and do not try to force her to get her into this thing just to fulfill your fantasy otherwise your happy married life may get devastate.