Hey I was in the same boat as you last year, I ended up with As so you deffo can change you’ve got time!
For bio and chem
Use PMT for past papers and when making notes
Miss estruch bio and allery chem are great for learning content, MAKE MINDMAPS!!
Do a past papers and turn the questions you got wrong into flashcards, and put the mark scheme on the other side of the card (I used gizmo online to make flashcards). At first it’ll seem like you’re getting loads wrong but trust the process! Before attempting your next paper go through your flashcards and brush up on any content you found hard. Do this for every AS and A level paper, then it’ll get to a point where you’ve run out of papers so start redoing them all again! Doing papers twice like this is actually quite helpful. It’ll help you remember those questions that get repeated A LOT and it’ll help you remember those mark scheme specific answers that bio needs. Each past paper you’ll start seeing improvements so don’t be discouraged! On the day of my actual exams I spent the whole morning blurting through my mindmaps and flashcards and mark schemes.
For maths… it’s similar.
Bicen maths and TL maths for the stuff I didn’t understand. I used maths genie for papers cause they had walkthrough videos too. So I’d do a paper, mark with the video and markscheme side by side, turn the hard questions into a flashcard. Repeat these questions like mad until I know the method step by step. Maths questions do repeat too! After I did all the papers I used MadAsMaths. When your feeling confident enough, use this as stretch and challenge, it has insanely hard questions that if you start doing them it’ll actually make the a level questions look easier to handle. Before going into my exams I would go through the flashcards and method sheets aswell.
I started revising around march up until my last exam, bear in mind I did revise like non stop so I’d recommend you start earlier so it isn’t as gruelling.
I hope this helps! These three subjects are tough so remember to take breaks every now and then. Good luck with it all I’m sure you’ll do amazing