Biology: Use the specification and make your notes according to it so you don't miss out key points. Use the endorsed board textbook and summarise from it - do not copy word for word.
Use YouTube to watch concepts that may be more visual, eg. the cardiac cycle.
When you are confident in your content - do LOADS of past paper questions. This is really important because without exam technique, learning content is pointless.
Maths: Examsolutions is a great website to go over content. Use this and the textbook to learn the content then complete loads of textbook questions. Once you're alright with the textbook questions do loads of past papers. Very important with Maths because questions are usually similar each year.
Chemistry: Same as Biology basically. Again use YouTube if you are struggling.
General advice: ask for help from teachers when you are stuck. Plan your revision out for the day. Start off nice and early too.
I got an A in Biology, and B in Chemistry and Maths (2 UMS off an A lmao) at AS. I was on AQA for them all and I currently do Biology and Maths at A2. (I didn't get all As but thought my advice could be of use)
Message me if you want more advice. I can also try and help you with Biology and Maths.