Gwen wouldn't be able to separate sex and love with Jack, I think. Though she did with Owen (probably because Owen is such a twat)

Rhys is awesome and doesn't deserve half the shit Gwen deals him. That conversation in Meat when he and Jack are in the truck really shows how much Jack likes him. I'm not sure he'd actually go for a threesome though. I think Jack's pretty aware of how much Gwen's infatuated with him and I think he'd want to avoid making it worse.