At my school at least, you could take A Level sciences without having done triple. There are probably only 1 or 2 people in my class of almost 20 that didn't do triple, and there are some topics that they've had to work harder to understand because the basics of them are covered in triple, but honestly I don't think you'll be at much of a disadvantage. As long as you do well in double, you'll be fine at A Level.