Hello zimzim40,
Because I do not wish to be a dentist when I am older, I am not exactly sure which Sciences you need for a place in a University but I would advise that you take Triple Science.
You have already said that you would rather get better grades if you take Double Science than get slightly worse grades when you take Triple Science but this is not the case.
In fact, from what I have heard, Universities usually favor students that have taken Triple Science regardless of what course they wish to study. This is because, even if you do get a B or a C in one, it is better because it shows that you are keen in Sciences even when you do not need it specifically for your job. Also, it can only benefit you since a B or C grade is still a pass and may be something extra to put on your CV in the future.
I would say, even though I have no idea which subjects you need to study at GCSE level to become a dentist, to take Triple Science. You may be thinking that it is extra work for something I don't really need, but it may just be the difference between you getting accepted into your dream University.
Think about it, and I wish you all the best,
- mossysilver