Ok, I've read Audi's RS4 safety features. They say that due to the seat structure, the RS4 complies with the side impact safety regulations even without side airbags, and indeed the curtain system covers the head.
Here:
"RS body-contoured seat
The RS bucket seat hugs your body’s contours so precisely that the official side crash requirements have been satisfied without the use of side airbags. It adjusts to your body shape, with inflatable lateral support and adjustable side sections, seat back and seat cushions."
The regular A3 (and presumably, the S3) shares the same principal of full-coverage curtain system, so I guess the S3 is the same story as the RS4, at least from Audi's standpoint.
Here's the bit about SIDEGUARD:
"SIDEGUARD®
In the event of a side impact, side airbags will activate, preventing occupant injury. The head-level airbag covers the entire front and rear side-window including the A-post area."
To my understanding, they are referring to the side airbags first, and to the curtain system in the second sentence.