It looks like there are 3 providers: A1, T-Mobile and Drei (3).
These are the three providers which operate the physical infrastructure for mobile communication in Austria. They rent their networks to virtual providers which sell communication services without having an own network. In fact there are about 30 providers you can choose from. Because of this high competition Austria is a low-cost country regarding mobile communication (e.g. as compared to Germany).
Whatever you buy, make sure that roaming and data roaming is included if you want to use it in Germany. Also make sure that you can top up your balance online via internet, should you be in Germany, where you won't get the reloading cards for Austrian providers.
Very popular is HoT which is available in every Hofer supermarket (the Austrian equivalent to Aldi).