advice for my choice of A-level courses

What kind of subject do you want to do in uni? If you are unsure you can always do 4 A-levels and drop 1 out at the end of the year.

I do maths, physics and economics and am looking to do aeronautical engineering at university, for context.

Agree with the maths bit, I enjoy it and don’t find it too challenging. I’ve heard from friends that bio is a lot of work, but you might be surprised to know there’s a decent amount of physics in it too. I don’t know how interested in space physics you are, but there’s a really cool area called astrobiology where you look at distant planets’ spectra to see their chemical make up and whether they could support life! Tbf, chem would be fine for that too I’m sure, but I just wanted to throw it out there just in case.
I’d personally suggest going for physics as your third as it pairs really well with maths and not terribly badly with bio. But it’s down to you, if you enjoy it, you’ll prob do well at it. Also, factor in what you want to do at uni and beyond.

Hope I helped, any other questions just ask

something on the lines of biomedical engineering or biophysics

If that's the case then physics as your third choice is a great idea. Since you are planning to take maths, these two subjects will reinforce each other really well. Good luck!