A triad is 3 different relationships. Other people are not communal property, she is your girlfriend and she is his girlfriend, but she is not your marriage's girlfriend.

Before you downvote me to hell, hear me out.

A relationship can only be between 2 people at once. You can try to coordinate so you're always having threesomes and always group texting and calling, but eventually you're going to need some one on one time.

My advice, stop trying for a triad (poly on hard mode) and just try finding yourself an actual lesbian girlfriend. You and hubby can share whatever his girlfriend wants to offer and everybody can be happy. Read the stories here of couples adding thirds and trying for a triad. Almost every single one fails catastrophically.