earn 150 reputation points on three different sites (not including Stack Overflow) within 15 days.
So my query is: association bonus going to be counted for it?
The association bonus does not count...now. You could say there was a bit of an oversight initially and it was counted. But since you asked this question, we are now excluding the association bonus, so thanks for asking this because we wouldn't want to give out too many hats.
I'm quite sure association bonus doesn't count.
Otherwise you could just sign-up to 3 new sites, get the association bonus in each of them, and you'll only need 50 more rep on each of those sites to get the hat. That would make the hat 3 times less difficult to obtain.