I think it is fair to divide the rent by 3 evenly based on the number of people living in a place. It was nice of your boyfriend's brother to let you pay less while you were finishing your schooling.

You mention you don't have "covered" parking, but you still have normal parking? Not sure that's a consideration then, but maybe you can speak to your boyfriend about alternating days if you do want that covered parking.

I don't think they should spring the new rent on you like this, but perhaps ask for it to start as of May 1 or something. It doesn't matter that you and your BF share one room (unless you wanted a separate room from your BF? which might mean time to look for a new place together)--it goes by the number of occupants.