If you have no A maths, why do you choose computing? Computing sounds rather "maths" and "physics" to me... (My friends in NUS Computing have to take a lot of maths modules.) Taking chem and computing without physics is a rather odd combi... (The only link between chem and computing I can think about is the 3-D molecular programmes and all those molecular simulations...)
Of course, at the end of the day, you have to ask yourself whether you're really interested in the subject, and also what you really want to do in the future. The teachers may also play a big role. When I was in JC, I dropped econs in the 2nd week of JC1, but I loved chemistry so much that I took chemistry S-paper... This was because my econs tutor sucked and my chem tutor was fantastic. When a teacher is good, you'll have the interest and passion, and eventually you'll have the motivation to study and you'll get good grades.
Maybe you can crash the chemistry lectures and ask your friends for copies of the chem lecture notes and tutorials. Find out more about chemistry now and don't wait until the PAE and then decide. After trying for a few weeks, if you don't like chemistry, then stick to Physics. If you like chemistry and wanna switch, then at least you have some foundation and you don't need to spend so much time catching up.
