I took Maths and Physics A'level and 2 A'levels in IT.
Something had to give due to so much coursework with IT and I dropped out of Physics, you might find some schools (like my school) won't allow you to take Physics A'level without it being accompanied by an A'level in Maths.
My plan was always to work in IT to fund my flight training, and should I fail to make it as a pilot then it's a pretty good back up. I went to University and studied IT also.
It seems the ATPL exams require a good understanding of maths and physics so an A'level in these subjects would prove useful.
If you have to pick 3 A'levels then I'd pick a subject your good at and that could pay for your flight training, Art / Drama / PE / Media Studies isn't going to pay the bills
.... Business Studies or IT should.
Good luck