there's an old walkthough i think that give you some triggers:
-After you've seen the late night living room sex scene with Susan and have passed day 90 Calvin will get the idea of trying to get Susan and Megan into a threesome.
- Late night threesome event. Once you've seen the threesome shower sex event you'll be able to choose between doing a threesome or Susan's regular sex scene during the midnight
and then the "end" at day 119-120 so you maybe want to make another save on day 110 or something like that, as far as i know there's 3 ends, one with susan, one with megan and the harem one.
i hope this helps
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this is the link in case you want to read it
https://attachments.f95zone.to/2019/04/297346_Cohabitation_Walkthrough.txt
the info i wrote there is on the "v 0.97" part