I do Physics, Maths and Chemistry, all with Edexcel and by far Chemistry is my hardest A-level, due to the sheer size of the content you have to learn and know for the exam. Maths is ok because it can be rope learnt just through doing past papers, and physics seems like a smaller syllabus. Also the things you have to get to grips with in chemistry are so much harder to understand than in physics which mostly makes sense. Although saying this, because I found Chemistry harder I ended up spending more time on it and will probably get a better grade in it than my other options (Not what I wanted).