Hi!
This is a really late reply so IDK I hope it's still relevant for you, but yes it is 100% possible to pick up 3/4 chem without having done it in year 11. I didn't do chem 1/2 at all, but at the end of the year realised chem was a prereq for the course I wanted to do, and swapped into it last-minute in year 12. It was definitely quite difficult at first to catch up, but I think it may be easier for you as you've actually done unit 1, so you'll be familiar with a lot of the vocab and jargon frequently seen in chemistry textbooks. Understanding and getting used to the chemistry jargon was by far the most difficult thing for me, jumping straight into 3/4 without 1/2, but it is doable!
I am currently doing Bachelor of Science, but my major is completely irrelevant to (human) medicine so I don't know how useful my insights would be. However, a lot of people I know studying Biomedicine (who aim to get into med) say that they would've chosen science instead if they could rewind the clock. Keep in mind that you do not need a pre-med degree in undergrad to get into MD. You could even do a Bachelor of Arts if you wanted to.