You have A*s in GCSE chemistry and biology and think you won't be able to do the A-levels in those subjects...? You need to develop some self confidence...
However I can see the argument that someone with only a 6 in GCSE Maths may find A-level Maths challenging potentially. Not impossible, and depending on what areas you did well in at GCSE it may well be fine, but you'll certainly need to put the work in and ensure you are very confident in certain key topics from GCSE (basically any/all algebra, graphs, functions, and trigonometry content I would imagine). Of course you may not need maths for what you want to do after A-levels.
While certainly helpful for most science degrees, outside of biochemistry most bioscience degrees you don't need it for admissions purposes and arguably will still be fine in the course itself without it. For a chemistry degree it's a bit more limiting though and there's more maths in a chemistry degree so that might be something to consider carefully. For things like medicine/dentistry/vet med, as I understand A-level Maths isn't required or preferred anywhere (except Cambridge as the singular exception to my knowledge) so completely unnecessary and any third subject would be fine.