What nonspecified stuff should I have done bbefore starting act three like having the fish girls or having molly living at your house?
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There's dozens of things not tied to the main storyline that you can do if you want to, but you don't have to - the optional/hidden stuff gives you more lore, extra characters, more scenes, but it's not necessary to complete the game :)
The list is too big to post here, but a lot of it is on the Wiki (link is on the main splash screen in-game) or on the CK Discord.