I assume you're in year 9 right now, right?
I was the same, and still am. I'm really good at biology, not as good at chem + physics but still okay enough to pass.
As someone who's in year 11 right now, my advice to you is to just study. It sounds like a rather mundane answer, but it's your best shot. Revise as soon as possible, but don't burn yourself out. Ask your teachers to go through certain topics, or even a friend/someone else in your class.
Even if you do double, if you get into higher (like I did) then you can get a B anyway, so don't stress. The main bonus for doing triple science is an extra GCSE, but it doesn't really matter in the grand scheme of things.
(Just a side note, in the end of year science test we did in year 9, I got a B and got into higher, and i'm certain many other people in my class currently got Cs in that exam, but i'm not sure if it's different for your school)