There isn't a polyamorous ending with Zenda. At most, you get the casual threesome and then Zenda makes it clear she's not interested in sharing further, and you continue her solo ending essentially unchanged.
If you want a relationship path with Maddie fully in it, you'll need to wait until their next game which should have the first Chapter out soon, where an alternate universe version of her is one of the main LIs.
I know you only mentioned solo routes, but wanted to clarify that Sadie can do butt stuff on MASH.