Has the OP left the building?
After all those pages of comments, probably. Still an interesting topic.
My 2 cents. As long as a committed couple can clearly share boundaries and can express their desires and fantasies there is the potential to work something out.
Personally, I think any kind of 3 some is a bad idea. On the other hand there can be "role playing" that "may" not cross boundaries and yet provide the "illusion" of a threesome. There is an interesting TED talk on Monogomish/Monogomy 2.0 and a "stimulus package or grey area of/for marriage."
In particular listen to around the 9:50 mark. Where she talks about adding forbidden thoughts to a marriage with no action. Then she talks about talk, but not touch. The brain is the biggest sex organ and it is easily stimulated and tricked. She gives examples of how to spice up a marriage and yet has the limits that they can live with. Then she talks about of actions without violating their sexual boundaries. For example if a 3-some is desired, then going to a "strip club (male or female), calling a phone sex operator, trying to role play some sexual fantasy.
A cuckolding 3 some fantasy, can be tying the husband naked to the bed, blindfolded, leaving the room, ringing your own doorbell, Having the wife carrying on a conversation with an "other man," playing a porn video of loud sex with the wife screaming out in her voice dirty talk. Then later having an imaginary conversation with their lover, and loudly opening and closing the front door. Then taking her time, removing her clothes, and opening the door to her husband tied up on the bed. She can then tell him all about the sex she had with her lover. She can then push all of her husbands mental buttons, whether it is tell him about her lover's size, technique, asking him to "clean her up" etc.