I think you'd be better off making this decision a little later when you have a clearer idea of which subjects you're excelling in, in comparision to others.
If you're really struggling with the workload, then drop down to 10.
However, if you feel that you're coping with the workload fine, then keep your units and see how you go. Doing 3 sciences is also advantageous as there is course content overlap.
Then maybe after the half yearly, reassess your situation. If you really can't see a reason to drop anything, then I suggest you keep them all - it makes you feel at ease knowing you have a backup, hence less stress.
Ultimately though, its up to you, do what you feel is going to be best for your HSC.