i did it this year and it was very VERY difficult.
i finished the unit for all three in the first week of may (about 5 weeks), studied for 5 hours a night, ever night, gave up my entire social life in those 5 weeks, (that includes even seeing my grandparents

), the stack of practise exams i had was about literally half a metre high.
the three midyears for bio, chem and physics, and ended up getting A, B+ and B, none of which im happy with and i know i wouldve done much better had i only had two exams to worry with.
the only benefits i found from this is how to organise myself for the end-of-years when i have 6 exams (two for methods).
how do you study for this? well if u want raw 38+ in all maybe im not the best reference (as by my results), but if i could go back and change things....
1: not do three midyears

lol, nah seriously u need to start early. do everything possible without a teachers help, so the first few chapters for bio and chem (i didnt do psych so i dunno) are all yr 11 stuff which is very do-able solo.
2: write your notes in the term 1 holidays. i found that the notes i wrote on summer holidays i didnt remember well because they were written 5 months before exams.
3: practise exams, practise exams, practise exams
4: every question where u lost a mark in a practise exam, write it out and memorise your mistake so that it doesn't happen again
5: go to church/temple/mosque/other-religious-place-of-worship
well thats all i can think of for now, sorry to be such a downer, i just wish someone gave me a reality check before i went into year 12 :/
im sure that you'll do your very best though, and in the end that's all you can ever do

good luck!