I reakon it depends on what you enjoy the best, and what you wanna do in Uni, if you have any idea. Alot of the sciencey type things in Uni assume you know Chem, but If you dont like it I wouldnt recommend it, it would be the hardest of the three, in my opinion; and I am doing all three. I find Bio the easiest,. but It really depends on your interests; like chemicalss and stuff like that pick Chemistry, like to know how living things work pick Biology, and if you like maths and learning about stuff sorta pick Physics, in the end if you pick soomething based on how it scales and you hate it it really doesnt matter, cos youll probably not do well in it, pick the one that you will enjoy the best, and work at it, your better to blitz something you enjoy that to come dead last and flunk something because it has better ranking...
Hope that helped
... I may have contradicted myself but oh well