This is an interesting topic. But while it has my interest
(because I'm a Chemistry and Biology person), I will decline from
advising you on this, because :
1) It don't really make much of a difference what your choice
is. Whatever you wish to do later (eg. medicine, researcher, etc),
you can still do with any 2 of the 3 sciences at 'A' levels
(but Chem is compulsory).
2) It really is up to you, what your interests are (physics or
biology), and whether you can cope with triple science or not (you
know yourself best), and most importantly don't end up
regretting a choice and say "it's so-and-so's fault. I shouldn't
have listened to him."
Hear from others.
And also, read the following entries on TakChek's blog :
About doing Science in Singapore
http://takchek.blogspot.com/2006/07/about-doing-science-in-singapore.html
Biomedical Engineering and US Medical School
http://takchek.blogspot.com/2007/01/biomedical-engineering-and-us-medgrad.html