Welcome.
But you need to be more specific in what you exactly would like to know?
As I'm reading this, I think you want to know if a 3.0TDI FMIC would fit an A3?
Or are you thinking of swapping engines(a 3.0 v6TDI in the A3)? If this last question is the case, then NO, will not fit without MAJOR bodywork and fabrication.
The first question depends on which intercooler you are planning on using(the 3.0 biturbo has 2 options) but there are easier direct plug-and-play intercoolers available for the 2.0tfsi