I am changing my hat on profile for three different days wearing different hats. I also earned hats on Thursday, Friday and Sunday. Still I have not got this hat.
The description of the "Hat Trick" for Winter Bash 2015, says hit the reputation cap on three different days
So you need to earn reputation from your post/edit on three different days. I.e., minimum 200 reputation on three separate days.
Screenshot for your reference: