most my a-level bio class have only done combined science. although i did triple science (ocr), my a levels are aqa so i don't get any of the benefits of doing triple :P
definitely is completely fine!! if this was chem or physics you would struggle a little but biology honestly isn't too complex. the main difference is biology has a lot more processes to remember, but the difficulty of understanding isn't as bad as chem/phys so the extra knowledge from triple science gcse isn't needed.
that being said, i would really encourage you to reconsider taking it if you don't get AA in combined science gcse (or at least A's in the biology papers), since it is an extremely difficult a-level.
don't worry!
(if you really want, after gcse you can self teach the triple science bio content but definitely not needed. good luck with bio, its mint!!)