http://www.sexinchrist.com/threesome.html
To summarize, we feel a Christian threesome is morally acceptable if it meets these conditions: It must be composed of one man and two women, all of whom recognize and maintain proper sex roles for men and women in and out of the bedroom. All married members of the threesome must consent to the arrangement and have consent from their spouses. And finally, the purpose of the relationship must be that it ultimately strengthens the existing bond between husband and wife and allows all three parties to share and celebrate their love of God together.