AFAIK no-one makes a 3" back box (or ever made one) for the Cinq.
Thank God, it was a later fashion which fulfilled the fantasies of Fat Car readers with Saxos and Subarus. It doesn't really do much for the noise, and on a N/A car probably does bugger all for power.
My T5 has a 3" back box (I didn't put it on) but it makes around 6 times the power of a Cinq.