@Laura4467 I wouldn't worry about getting a bad review. If the host does give you a bad review for this, you can simply tell future hosts who question it why you got a bad review. Any good host would understand that it was not you but the host who wasn't playing fair here.
Because the fact is it's strictly verboten to change the price after a confirmed reservation (https://www.airbnb.com/help/article/1505/can-i-change-the-price-of-a-pending-or-confirmed-reservatio....). All you have to do is decline the host's request.
A few other notes of clarification:
1) You should know that Airbnb, not the host, has the deposit. The host won't receive any funds until after you check in.
2) You are always able to see any listing's cancellation policy near the bottom of the listing's web page.
3) It is possible the host didn't deny the request for a third night - the site can be glitchy.
I think it's dirty pool to try to change prices in this fashion and you're right that he had plenty of opportunity to set those new prices. I think you should deny his request.