heyyyyyyyy!
i do all 3 sciences, accelerated 4u math & extension english.. i think that you can definitely do it, if you want to.. but i am intending on dropping physics ASAP as its SO BORING [imo].
biology is quite easy, the concepts are simple to grasp and there are only a certain number of syllabus dot points that can be examined, and to be honest with you.. there arent that many
workload is very low, i have a good teacher and its easy enough to learn everything in class and then revise before exams & still have a top rank.
as for chem, the dotpoints are so much broader and there are heaps of other skills involved. i personally find it the most gruelling science, and most of it you simply have faith in, as there's no way you can actually prove alot of things you're taught. keep in mind that at alot of unis chem is assumed knowledge for heaps of courses [ie medicine]. the workload for chem is really high... for me, anyway...
physics, as aforementioned: is DRAB, dreary, monotonous.. alot of it is just basic algebra & boring stuff you will have learnt in junior science [waves, electromagnetic spectrum, electricity stuff] -shudder- many say that it can help with maths later on [i beg to differ], but often with physics the questions are quite convoluted...
i dont regret doing the 3 sciences at this point... as i know they will all come in handy later on, but as someone said earlier- it gets really repetitive & i personally think that i [& anyone else who does zee 3 sciences] need more balance in my/their timetable. in hindsight, i would have kept with software instead of physics if the teacher wasnt so scary.. but yeah, i know a few people who do all 3 and they absolutely love it, so i guess if you have a passion for it, you look forward to science lessons as it is- then yes..do all 3.. remember that in prelim u need 12 units, so science is going to take up 50% of all ur subjects if u do all 3!! its certainly not social suicide, or anything like that- just dont do them if u lack any fundamental skills coz then you're disadvntaging urself and the rest of the class.
but i think you'll know what's the right thing to do so... good luck with your decision