When people ask me about what a-levels to take i would always say do the ones you enjoy. Do the subject where you like the teachers if you know them. It makes a huge difference.
For nursing, some universities want atleast one science subject, often biology is preferred, so take biology if you want to, but if you hate it, don't. Biology is also really hard if you're not willing to work: I have 12 above C GCSE's and I really struggled in biology.
The reason for taking subjects you like is that you have to revise. Revising is a must at a-level and if you don't like the subject, you won't revise it as well as you should so therefore don't do as well. Apart from the biology for some universities, most just want any 3 a-levels, and as long as you get above a C in all of them, you've got a shot at getting in.
I would also advise taking at least one applied subject, but don't take 3. Applied subjects are generally easier but contain a lot of coursework, so hence taking 3 is a bad idea, too much coursework!
Good luck in what you choose!