Did I ruin my chance of a good atar?

Keep in mind that the SEAS process (given your sickness) might mean that even if you do not quite get a "good" raw ATAR, you still might be accepted for your degree. As always, there are alternative pathways.

I suggest making sure that you are up to date and confident with all the content so far over the holidays. Attendance rate itself is only important if your school requires it to pass, so check with your teachers if you are unsure. Your SAC average depends on your ranking in your class and the strength of your cohort. I would say that if you improve substantially for Unit 4 SACs and the exams, you are in with a solid chance for above 80.

Belle22 Remember Unit 3 contributes to a very small part of your study score, especially for maths subjects. There's no point wasting energy in worrying about unit 3 sacs, it's not gonna change anything. Give it your best for unit 4 and the exam and you'll do well.

I was in the same boat for chemistry in which i flunked unit 3 for my standards with an average of 85% and rank 5-7. I did very well for unit 4 with 100% sac average and was promoted to rank 1 overall and finished with a study score of 43. If I pondered over my unit 3 results this would not have happened.

Don't leave any regrets, just continue working and you'll get the atar you want.