I presume you have little problems with maths and humanities
since you didn't mention them.
The trick to scoring is what you are currently doing, questions
drilling. You can do it open book during practice as well. But
questions drilling also requires you to throughly understand the
question itself, understand why the answer is done or calculated
this way etc.
Since you love the 3 sciences, you should continue on as pure
sciences. As per a discussion earlier on, when taking combined
science, you are actually doing more work/studying per subject than
taking 2 pure sciences. Only thing is, you will find it easier to
score because you compete against students who are not that good in
science as well.
My strategy, if I were you, would be to continue with the pure
sciences. Assuming you score well for pure physics (A1) and pure
chem (A1), and ok for pure Bio (B4), it's to me a better score than
if you score well for pure physics (A1) and combined science
chem/bio (A1).