Well there's a big difference between Physics and Biology (the way it's taught etc.); surely you know which you prefer? If not, then just try them all, if you find out that you dislike Physics (it's different to GCSE) then you can simply drop it - the same applies for the others.
If you are taking Physics, you will also need to take Maths. I haven't heard of many people that get an A/A* in Physics without taking Maths - it's also a requirement for a lot of courses (scientific especially).
Furthermore, if you're a top student and would like to study something along the lines of Physics at a top university then they will ask you why you didn't do Further Maths (assuming it's offered at your sixth form). Further Maths is something to consider if you think that you may end up going down the Physics route.
I'm not doing Chemistry and Biology in September so I will not be able to give you the best answer, but from what I've read they're also difficult.
If I generalise the feedback I've been given, then Chemistry appears to be the hardest out of the 3 (of course it's subjective to the individual), Physics is just behind Chemistry and apparently Biology is the easiest but it's still tough.
Biology - Are you good with names and remembering things? This is a huge part of Biology, you won't need to do much application/problem solving (compared to Physics and Chemistry).
Chemistry - This is a mixture between remembering concepts and applying that knowledge to a question. The maths is fairly basic; you can get away without taking Maths (the same can be said for Biology).
Physics - Like I said, Maths is a must to be honest. A lot of AS Physics appears to be the application of formulas. You don't have to remember as many things as Biology and Chemistry, but you do need to know how things work/concepts behind them. A personal opinion of mine is that you have to be more organised with Physics; as opposed to the other two, you need to remember the correct units and how to display them and which formulas are required to use (SUVAT etc.)
You should be able to manage all three of them and Maths, I know a lot of people who have done. Just consider Further Maths if you think Physics could be a career option.
Summary; try them all, if you don't like one then you can drop it. Simple.
*I apologise if any of my generalisation of Chemistry and Biology is wrong, I'm not studying either of them in September so I could be wrong; I'm just basing it on feedback.