Just finished these a levels last year and currently tutoring people on these subjects as well😂
So don’t panic, and you will progress bit by bit to get there.
So for Biology, start with understanding your concepts(youtube videos helps for me) then try to remember it. I like blurting, basically you just write down everything you know, then compare it to what you need to know. Writing is positive reinforcement and also this process help you to actively retrieve info. Once you’re satisfied with your knowledge, you go on to questions and try to gradually tailor your answer more specific to the markscheme. Also most of the time you can bullet point which helps to keep it concise.
Chemistry and maths : a lot of it is just understanding and application. Once you understand it, you are half way there. Then practise questions to apply them more. For chemistry, its always nice to remember your whole organic synthesis as well as definitions. Resources like a level maths revision or dr frost are nice, if you want more challenging ones prolly madas maths.