Some skills are common to all three sciences. It's very common to be asked to draw and interpret graphs (including with 2 or more sets of data being displayed and going through the x axis). If you can work on describing trends, interpreting data sets (including from tables) and drawing graphs, this can give you extra marks across all three papers.
I'd also suggest making sure you're really clear on key working scientifically terms (e.g. reliability, accuracy, repeatability) as again, these are often asked about across all 3 papers and if you're very clear on them, you will pick up extra marks.
I'd also consider focusing on topics that you consider yourself to know a bit about but not everything (e.g. you'd get 2/3 marks on a 6 mark question). It'll be easier to improve your knowledge here than starting to learn things from scratch, and there's not much point spending time on things you're fairly confident on.
I'd also suggest learning as many key words and definitions as you can.