I use a Test Kit to measure SO3 and IC to measure SO4 cause, IC can only measure SO4.

For sulfites I have performed an acid distillation then titration with a known concentration Iodine solution.

Thanks Micheal and Bruce for your useful answers. I think  I will try it out with IC.

If you only have SO3 and SO4 in solution you can use UV to quantify the SO3.