depends on the tournament. i don't know how it's in actual HS volleyball in japan, but at least in this manga so far all the 3 set matches were payed for the full 25 points. a shortened final set (up to 15 points) only appeared in 5 set matches (karasuno vs. shiratorizawa).
1-2 more chapters and I can finally breathe... Isn't the 3rd set usually 10pts?