Start revising now, start slow and steady. Find a PLC sheet for the subjects online and identify weak topics and strong topics so you know what needs more focus. Complete questions on strong topics to make sure you’re still strong and obviously revisit weak topics. Free science lessons and loads of exam questions are readily available online for science, consider using flash cards as well for learning all the content - exam practice is the most crucial for getting top grades. I’d suggest little and often - for example, revise monday - cell division for biology, caluclations for physics, Tuesday - ecology for bio, structure and bonding for chemistry, Wednesday - waves for physics and energy changes for chem etc etc . By the end of the week, you would’ve revised each subject at least twice and built on weak areas for all 3, and doing this throughout the year then intensifying revision after Christmas will work wonders