Enough preliminary content so that you know what's going on. Prelim content can only cover at most 20% of the paper so the bulk of the paper will be those 3 specific topics. You should be prioritising those.
So do any past papers your school gives you, then find your own past papers (from various sources such as THSC) and do all the questions that are relevant to prelim, or the 3 topics in question. If you happen to find a half yearly paper that covers the exact same thing your's is covering then that is excellent. Else, there is nothing wrong with plucking specific questions out of trial HSC papers.
(Of course, if maths is the only thing you have to study for, then you could put enough effort in to have a solid coverage of the whole course. But I'm assuming that won't be the case.)