I just feel like Luke now has his guard up where Nate is concerned, after the blackmail. So a threesome is less likely, though still possible. As
TonyMurray said it will probably be through coercion of Luke and the situation rather than seduction. Could also heavily depend on how far Luke/the player allowed things to go on the bench and if Kath chose Luke or Nate for the blowjob during the photoshoot.
Sadly Kath seemed to believe Clem when she lied (depending on choice) about Luke touching her in the shower, Kath just didn't bring it up with him as she didn't want to risk losing him. Would she also believe Nate's lies about what he may or may not have seen in the closet between Luke and Claire?