Hello Malfa,
Please put your country of origin in the location section of your profile. It helps forum members hand out appropriate advice.
I don't know anything for sure about the S3, but I do know that the smell of gasoline fumes (especially in the trunk) is an ongoing problem for many spider owners, and as far as I know there isn't one cure-all solution. It seems that every owner has found a unique treatment for the problem, and some of them work better than others.
With my car, it turns out that the fuel vapours were being vented inside the car. I have no idea if problem was unique with my car (for we all know that Alfas tend to be a bit quirky in their assembly) or a "design feature."
If you want to see other causes and solutions, head over to our American cousins at the Alfabb.com and see what they have to say about fuel smells. There are pages of threads devoted to the topic.
--Toronto