Personally I would recommend you to have 11 units rather than 10, this gives you a bit of buffer since only your best 10 units are counted. If you aren't that good at it, but it isn't taking up too much of your time then keep it, it might turn out better than you imagined, and if it didn't? Well, it wouldn't be counted. However, if it's taking up alot of time and it's making you feel disheartened about the HSC, then do drop it.
Don't drop it because you think "I only need 10 units, why am I wasting my time doing 11", because not only do you not get the experience of more challenging and interesting mathematical concepts, but it may cost you that degree you wanted. I took 15 units in my HSC year (ok, I dropped chinese background just before the trials, so it should strictly be 13 units) and I managed fine, although 4U maths took up almost all of my time.
Oh, and another thing, 3U maths scales better than many of your other subjects, in fact it is probably the highest scaling subject on your list, keep it unless you really are struggling. (struggling = getting 50% for exams, anything higher than that you can improve on without too much trouble)
Anyway, hope that helped, and best of luck!
