How long will that Underwood threesome last?
First of all, rest in peace to Edward Meechum, the real threesome victim in all of this.
Frank and Claire seemed to have found his replacement in Thom Yates, though. He almost had a little fling with Frank last season, but he went full force with Claire in Season 4, building a connection with her during the time her mother died. He was there before, during and after it all happened. It was a very emotional moment for her, so it makes sense why she started seeing him in a different light. Not to mention, her mother approved of him.
From there, things actually turned into sex (then love) and the two played their little game until Frank figured things out and accepted their relationship. But will he join in?