Well assumed knowledge for pharmacy at the University of Sydney is Chemistry, plus either Biology or Physics, plus Maths. So you don't necessarily have to do all 3 sciences to help you with pharmacy.
and there is always the possibility of doing bridging courses at uni.
but yeh, doing chemistry would understandably have a huuge advantage. keep in mind that doing all three sciences will be demanding nevertheless, and may become to much if you dont keep up to date and understand everything as you go along. u can risk to fall behind.
Actually, it is easier to do three sciences as all three link together somehow .. In fact, I probably will not know which topic belongs to Physics or Chemistry, half-life for example (not the good old 1998 game)