I know how you feel. I'm doing fine in Chemistry and Physics, but not so well in Biology. I want to drop Biology in Year 12, but then I may be facing problems with study units. I can safely say that I am in the top ten or top twenty for all my subjects at my school.
If you are aiming for Science in university, then your study would be worthwhile. The topics you learn are relevent what you will find in university science courses.
Also, with the scaling of these science courses, it wouldn't be too bad to keep some of them in Year 12.
For me, I can't stand writing a practical experiment report(s) almost every week. The school's policy is that if you don't get at or above the B- grade for your prac report then you have to redo the whole thing again!
If you wish to do 4 unit Maths, like I am, I think you should consider dropping one of the sciences.
Despite the work, I find all of them rather interesting. The beauty of it is, that the courses overlap each other at some points. For example, nuclear reactions in the Preliminary Physics module Cosmic Engine, can be applied in nuclear chemistry in the Chemistry modules. Also, learning about chemical reactions in Chemistry can be applied in chemical reactions of living things in Biology.