Matt seems more like the likely candidate for a mutually agreed to threesome if one or the other are reluctant. In one path Nat was messing around with him in club, and he keeps messaging her, so if she shows Ethan her phone conversation with Matt She could convince him to invite Matt over and Matt wouldn't mind sharing because he's on their tuff and IMO he's young, horny and doesn't care about the cuck-games. Not forgetting that fits in with two or more different scenarios.
As we already know the office girl will be Ethan's potential consort and be Nat's foil.