You need to frame this situation as if it's your wife's fantasy. You know, "wouldn't you love the feeling of a woman licking you while I'm fucking you, etc". Very carefully sub-communicate all these rich descriptions to her, NEVER directly verbalize any of it (ie. "wanna have a 3-some? heh). Then you should set it up so that it's something that "just happened". Nobody is to blame, it "just happened". Good luck!
