Contrary to popular fandom belief, I'm afraid threesomes are not the solution to everything.
Actually seriously though, that would be a pretty weird dynamic. Dick's way too young for Dinah, and they know each other mostly through slightly awkward connections (on-again-off-again's top agent who she cries about me to, coworker's son & surrogate son's best friend, woman who works with my dad, guy the most important woman in my life is hung up on), and they would only be doing it for Barbara's sake. Kind of an interesting challenge, actually, to genuinely capture that - two women doing it for a watching/directing man, who clearly have no authentic interest in each other and all the tension is directed at the voyeur, is a relatively common device, but I've never seen a man and woman paired who clearly have no authentic interest in each other and all the tension is directed at a female voyeur. There's always a suggestion of genuine desire in any het contact in porn, in my experience. (Even as a writer.)