Although prelim counts to absolutely nothing, I'd rather walk in the doors of the HSC competition with some sort of confidence that since I did well in prelim, I'll be doing well in the HSC 
Uh...2? Right now I got 2 assignments to do and they're both due next week
Just to add my two cents: I personally got into really good habits during Prelim which I was able to carry through into the HSC e.g. doing my summaries and practise essays etc.
But I can clearly remember this one afternoon in the lead up to my Yearlies when I had a kind of epiphany and I realised that
there was only so much I was willing to do for an exam that didn't actually count. So I did an average amount of study, no all-nighters or anything crazy like that, and I ended up really happy with how I went.
Moral of the story is:
1. Don't lose any sleep over Prelims. Seriously. Just don't. (Hopefully this has already been reiterated enough)
2. Finish your assignments to the best of your ability and fit in extra study when you can. 3 weeks is more than enough time!
3. Decide your breaking point/maximum effort and stick by it no matter what e.g. no studying after 10pm
3. Don't have a break down over Prelims its honestly not worth any of the stress/tears/anger/anxiety. You're going to have way more important things to worry about next year.
The main lesson you should be learning from Year 11 is what study techniques/patterns work best for YOU so that you can implement them when it counts.
And most importantly, start by taking the advice from all these people who have come before you 