In three months you can study a lot of material. You can find a good calculus book and you can find out come good calculus books in the website goodreads.com(it contains reviews by users). Then you can re-study the material that you covered in your last year but not in the traditional sense. You must just try to solve a lot of difficult exercises(i take for granted that you know the basics very well--if not, then you must re-study the material from the beginning in the more traditional way). As you don't have a strong background in mathematics, i do not think that it is a good time to study mathematics that are taught at first or second year in universities. I think that you must just try to build a very strong background in your last year mathematics. Also, make sure that your background in physics is REALLY solid. If you want to succeed, you must know the basics perfectly, not just well. Also, there are video lectures in the internet for first year calculus(try youtube). You should be a great problem solver(in both mathematics and physics) and also get the intuition right. The university builds on that knowledge and just gets more sophisticated from there and on, so your base should be very strong because your intuition will build up when you start connecting the more sophisticated stuff with the more simple that you encounter in high school. Lastly, just in case that your high school teacher taught you the material at a lower level than other high schools(you will compete against students from top high schools), check out what other people are taught. For this, the internet is your best friend.