The term really applies to husbands who have wives that cheat on them behing their backs. In an open relationship with a third member, I don't think cuckhold is the best term to apply to the husband. Often he receives much pleasure as well, whereas true cuckholding is very likely emotionaly painful for the husband, as it would be for a wife whose husband is cheating on her. Lover, other lover, larger lover, our bed partner...all seem more appropriate.