Oh, thanks for the clarification. If this is a story without a fixed protagonist, but simply observing it from the outside, then everything is much easier.The story is largely told from a third person perspective, so it depends on if you choose to identify with any of the three characters, and if so, whom. If you identify with the husband, it's an NTR story, the wife, cheating, or the "third party", netori.
If you don't identify with any of them but just follow their story, any or all of those could apply I guess.