Well I learnt the entire RS course the morning before the exam so ofc you can. GCSE physics requires very little actual physics so you should be fine as long as you understand the basic principles and maths is fairly straightforward too as the papers have almost the exact same questions year on year. Chem I can't speak for since I remember literally none of the course I reckon an A* should be possible since the A* percentage is remarkably low for Sciences so long as you complete as many past papers as you can before the exam. Hands down past papers are the best way to revise, it seems shitty at first but after 5-6 papers you begin to notice that there are only certain questions they can ask.