As far as I'm aware, it's not exactly just 3rd highest HSC mark, etc. - I'm gonna copy over my explanation from another thread:
How it the averaging actually works to the best of my knowledge:
1. You get your internal ranks
2. Say the highest internal mark was 60 and the lowest was 20, and the highest in the HSC was 80 and the lowest was 60 - the highest rank internal will get 80 as their internal mark, and the lowest rank internal will get 60 as their internal mark (externals are not affected)
3. If you're in between, then you get moved accordingly! So if you say ranked 2nd with a 54 internally, that would probably get changed to approximately 77. Note that though your actual internal mark doesn't do anything, it still determines how well you moderate, so it's not purely rank that matters.
4. Then they average this score and your external mark!
Take this with a grain of salt because I don't know how accurate this is, but I just wanted to put this here!