I got curious, due to what happened on last patches (I don't want to spoil anyone), how far is the game from the end? Also, does the dev ideas for next game.
I'm sad the dev won't have a threesome, maybe an ending wouldn't be possible, but a scene would be nice at least. Still, I understand the decision. As long Cassius continues to be a good boy.
By Zorlun's indication, chapter 4 is only at the start. He had once indicated he was thinking that game would finish +/- at the end of this year.
He did also say he has some ideas for another future game.
Threesome there are, with Marc and Katherine not possible.
From one side for the "no Katherine corruption" rule, though admittedly that could depend on how one would define "corruption" (e.g. if
Marc was abusive and Katherine and Myriam submissive I think would be considered as corruption of Katherine, considering how protective of Myriam she is, but if it was Myriam "showing her love" to Katherine and then Marc arrived and joined by going for Myriam in a way that does not trigger Katherine, it may not or not be considered as corrupting Katherine, depending on whether anything between Myriam and Kate is by itself considered as corrupting Kate ).
From the other, and that is the one that if not eliminated, severely restrict what Zorlun can do despite being the author, there is the Patreon issue. No explicit sexual activity between Myriam and Katherine possible, is what Patreon told Zorlun.
It is something I cannot even go around with the incest patch, since Katherine was always, and still is, even in the Patreon version without incest patch, Myriam's daughter.
About the endings, actually, it may depend on the "Katherine no corruption" rule, but also on whether Zorlun can afford to risk Patreon's rage for the next game, but it could be the easier way around.
If he considers that any "activity" between Myriam and Katherine would be corrupting Katherine, then "no way". If that is not the case, there is still the Patreon issue.
If the "Kate no corruption" rule does not block, if he needs the guarantee to continue relying also on Patreon for the future, it can still be a "no way", to avoid risks.
Otherwise...
Maybe, but it is only a maybe, there could be in one of the endings, eventually if Prometheus (so, not Zorlun) integrates it in his patch, something like Myriam having, if not really a threesome, at least something with Katherine and Marc, if it can fit somehow in the story/ending, though again, given the relations, would seem more probable something in which Myriam "show her love" to Katherine and Myriam, but not something too hard).
But keep in mind that as far as I recall, Zorlun had said he had already in mind the possible endings, so my "spoiler" is more something to titillate
your fantasy, than something that may really end up in the game.