I did AQA for chemsitry, physics, geography and edexcel for maths. For AQA, chemistry is in my opinion far easier than physics - I was looking at getting a high A but made a bit of a mess of my coursework, so will probably get a B. I can't really speak for biology, as I don't take it. All three sciences will be tough, but it is manageable. I know two people who are doing all sciences and ones doing maths as well, and they are both managing just as well as anyone else. So I would say three is fine as long as you're decent at science which it seems like you are.