i'd imagine that the S3's insurance will be more than the 2.0T's. but again in context the car is actually the most expensive to insure of the 4. granted one of the S-Classes is a 2000 model and is basically worthless, but the other one is a 2 year old 320 CDI with a full AMG bodykit and quad AMG exhausts worth £80k brand new and the SLK is only a 200 but it's got the Sport pack on it so it's fairly flashy.
the point i'm trying to make is that the A3 is expensive to insure regardless, but i'd imagine that if you want an S3 you're gonna spend more trying to make a TDI look like an S3 than if you just got insured on an S3 in the first place.
btw i'm 23 years old and i had to declare 3 points for speeding and have had 2 accidents in the last 3 years. both weren't my fault but it still makes a difference. however i'm also a doctor which brings my insurance down a bit.
the only other tip i can give is that having a girl on your insurance on addition to your fine self, your dad and bro will actually lower it. your mum, sister, cousin it doesn't actually matter and it doesn't actually matter if she drives it or not but it'll still bring your insurance down.