There were no indications that the two Iraqis and one Syrian, arrested in refugee centres in the western state of Tyrol, were planning attacks in Europe, local police chief Helmut Tomac said.
The 27-year-old Syrian is believed to have fought for Al-Qaeda's Syrian affiliate, Al-Nusra Front, and killed 20 members of government forces, including prisoners, local official Peter Oehm said.
All three arrived in Austria last year, part of the massive wave of migrants who travelled up from Greece through the Balkans into Austria and beyond, police said.