From www.sexinchrist.com/threesome.html :

"Playing by God’s Rules

If, on the other hand, a married couple feels their relationship would benefit from them establishing a loving involvement another woman, out of respect for the couple’s marriage, and out of respect for any marital attachments of the other woman, they must abide by certain limits and conditions:

(1) To avoid the impropriety of male homosexuality, a heterosexual couple should not under any circumstances form a threesome with another man.
(2) Both women involved in the threesome must be willing to keep within traditional female roles (i.e., not taking on masculine appearance or behavior in or out of the bedroom) and recognize the male as the leader in the relationship.
(3) If the wife’s lesbian sex partner is unmarried, it may be permissible for the husband to have relations with her only with his wife’s consent.
(4) If the wife’s lesbian sex partner is unmarried, but the wife does not wish her to have relations with the other woman, the husband should respect this.
(5) If the wife’s lesbian sex partner is married, her husband must not have objections to the relationship.
(6) If the wife’s lesbian sex partner is married, the husband should refrain from having any sexual relations with her, and should make every effort to control his fantasies about her. He should concentrate his attention on his own wife..."