Hey there - I do all of these and enjoy my subjects immensely, so here are a couple of tips to succeed at A Level!
For maths, you want to be practising little but often - maybe go over this at the weekend, and do a load of questions. Good resources are PMT, maths genie, and if you have a textbook, definitely use that! Biggest tip would be not to ignore applied maths (statistics and mechanics) as, even if you got every single pure question in your A level exams correct, if you got no marks for applied, that still wouldn't be enough for an A* !
For chemistry, notes and questions are the way to go! Check out some videos on YouTube to help you understand the content.
Rather controversially, I don't think physics is the hardest science - if you're mathematically able, you've already got loads of marks in the bag! Yes, there is some theory, but much less than the other sciences in my opinion. Do loads of questions for this one, and, again, YouTube videos will save you, especially for those difficult concepts.
Best of luck!