The exterior roof trim piece has stress cracks on each side of my Audi A3-

Hello, have a local shop near you or dealership evaluate the cracks. Nevertheless, the components will likely need to be replaced at your expense. Obviously, I cannot see your trim, but I can share that one common factor of trim failure and damage is related to the environment. Extreme sunlight and reflective sunlight can damage plastic trim pretty easily and quickly. You may want to review where the vehicle is parked - when not in your carport, or even within the carport if there is reflected light causing an issue.