The A3 was designed as a 3-door. When they developed the 5-door version, they called it "Sportback" instead of "5-door", that is all there really is to it. "Sportback" sounds less boring than "5-door", not so? Why people up to this day wonder if it is a station wagon, and even confuse it with "Avant", is beyond me. It is a normal 5-door hatch, like a VW Golf.
A real stationwagon, is "Avant" in Audi language, i.e. the A4 Avant or A6 Avant.
The A5 replaced the A4 coupe. Then they decided to develop a 5-door version, so once again...calling it a Sportback.
The A7, which is based on the A6, is called the Sportback, maybe to align it with the A5 Sportback in terms of number of doors, or maybe to make room for an A7 coupe (3-door) in future...speculation on my part.
Criticizing VAG for being crazy with their models and naming conventions are a little bit unfair, look at BMW for example - they have the 6-series and then a 5-door version of the 6-series which they call "Gran Coupe" - but "coupe" means two-door, right?