It's pretty clear to me that you still don't have a definitive idea of what you want to do. This isn't necessarily a bad thing, but I'd advise you to spend the next 2 years honing in on your interests and talking around/doing work experience. Put frankly, if you're jumping around from 3 bachelors to psychology and engineering, it'd be apparent that you're still very open to a huge array of courses. You don't have to pick now, and it would probably be better if you didn't prematurely lock yourself into a pathway.
HECS is a Uni payment scheme by which students in Australia can defer paying off the fees of their course until they have a job above a certain wage theshhold. Put simply, you avoid paying for uni in the short term, but it catches up on you eventually.