I’m a successful offer holder for chemistry (though my third A level is physics and I’m doing a level FM and AS biology in my gap year). Not quite who you wanted to hear from, but here’s my take:
Having been to an open day and heard from a number of people currently taking chemistry at Oxford, a number of them have mentioned that their third A level was not a second science or FM (English Language appears to be quite a common third subject among successful applicants for chemistry).
I reckon economics is something you could use in your PS to stand out just that little bit more. After all, doesn’t economics require you to be able to present ideas to an audience and defend your arguments against scrutiny? Surely this adds to your skillset rather nicely because later down the line you’ll have to defend a thesis as part of your degree.
You could take an AS science in year 13 but I wouldn’t recommend it. It will just distract you from your other subjects where you need to be able to get A*A*A at least with the A*’s in chemistry and maths. You could take AEA maths (ask the head of your maths department about it), as this won’t require more lessons outside of your normal A level maths classes.