To start with, they may not be ripping you off so there's no reason to jump to conclusions. By the sounds of the information you have given, they have replaced a cam cover (rocker cover) gasket and a '3in' gasket (which could be a 2 bolt 3 inch flange gasket for the cat etc). The gasket between the turbo and manifold is reasonably common to fail in these anywhere around 120k. We do one around every month or two so I would say it's the most likely cause. If it's a cooler pipe making the noise then it could be from them removing it to gain access to the cam cover gasket in which case, they should fix it....but.....if they did a 3 inch gasket at the cat converter and a cam cover gasket, then no, it is purely coincidence that the turbine inlet gasket has blown.
Certainly no reason to give them the flick straight away. Take it back to see what they say, if they say something technical, get them to put it in writing and post it here. I am fluent in tech speak.
SAU won't let anyone rip you off