hello raelubsfoxes!
the way to get the third ending is to start off with reasonable answers and get less than 20 beauty points!
after that aim for the more ridiculous choices (new ones will appear!) until you get Ending2: Significantly Sadder, it needs more than 20 points, so if you got it on your first run through you might need to reset and fight the urge to have more treatments xD
sorry about the troublesome method, that's how the game was programmed ;-;
the first ending is Mildly Mortified (less than 20 points )
the second ending Significaly Sadder (over 20 points)
The third ending (after getting end 1 and 2) and going though some of the new choices!
after getting both endings you can start a new game and you'll be on the third and last run, and depending on your choices you will find the last ending ^^