If the third ending is closed off, getting it requires starting a new game.
I'll give a hint: in the ending area, what seems to be blocking the "exit" door?
First of all, I feel honoured to have my comment answered by the studio itself and secondly thank you so much for the hint. I thoroughly enjoyed the game and all its endings ( the third being my absolute favourite), I loved the music I've been listening to it for the past 2 days. The sprite work is fantastic and the overall game is an excellent homage to old-school beat-ups. I cannot wait to see what the studio has in store for the future, nsfw or otherwise.