I did A level human biology (i also have a degree in it) and am now finding it quite relevant to some of the ATPL studies. I didn't however do A level maths and now wish I had (though I can still cope with the mathmetical nature of ATPL with GSCE maths and the maths I picked up at degree).
Ultimatley go with what your good at and what you enjoy (as everyone else has said).... but if you don't mind me saying 3 a-levels in drama, art and religion or something like that probably won't be the most useful for a career in aviation.