U3, an underground rapid transit line in many major German/Austrian cities: U3 (Berlin U-Bahn) U3 (Frankfurt U-Bahn) U3 (Hamburg U-Bahn) U3 (Munich U-Bahn) U3 (Nuremberg U-Bahn) U3 (Stuttgart Stadtbahn) U3 (Vienna U-Bahn) Avies (IATA code: U3), an Estonian airline Cessna U-3, the military model of the Cessna 310 aircraft German submarine U-3, one of several German submarines Boeing Customer Code f