Hey mate, unfortunately you won't get marks for that. Full marks on the exam means full marks on two of the three sections - you don't get bonus marks for doing more.
I know your exam is done, but for your future exams, please do not do this. Best case scenario, you wasted time on the exam doing questions you won't get marks for. Worst case scenario, the examiner marks the extra section you did and ignores your answers from the sections you had actuually studied for.
Examiners are only expected to mark 2 sections and won't waste time marking more than that.
Since the ancient history exam is set out as Egypt, then Greece, then Rome, there is a chance an examiner would see you wrote answers from Egypt and Greece and wouldn't bother flipping to Rome to check whether you had also answered those questions.
Anyway, not trying to freak you out because examiners are quite lenient in VCE and will probably realise your actual answers were for Rome and Egypt. But no, you definitely won't get bonus marks.