The Germaniawerft engineers refined the design's hull shape through extensive model trials. "U-3" was long by abeam and had a draft of . She displaced surfaced and submerged. She was armed with two bow torpedo tubes, and was designed to carry up to three torpedoes. After "U-3"s arrival at Pola in January 1909, she was commissioned into the Austro-Hungarian Navy on 12 September 1909 as SM "U-3". During the evaluation of the "U-3" class conducted by the Navy, the class' poor diving and handling characteristics were noted. To alleviate the diving problems, "U-3"s fins were changed in size and shape several times, and eventually, the front diving planes were removed and a stationary stern flap was affixed to the hull. "U-3" served as a training boat between 1910 and 1914 and made as many as ten cruises per month in that capacity. At the beginning of World War I, she was one of only four operational submarines in the Austro-Hungarian Navy. On 22 August 1914, "U-3" began operating reconnaissance cruises out of the naval base at Brioni, but moved a month later to Cattaro. In April 1915, a quick firing (QF) deck gun was added. On 10 August, "U-3" departed from Cattaro for what would be her final time for a patrol north of Brindisi. Two days later, while returning to Cattaro from the Straits of Otranto, "U-3" launched a torpedo attack on the Italian armed merchant cruiser . The torpedoes missed their mark and, in the ensuing action, "U-3" was rammed by "Citta di Catania", which destroyed the U-boat's periscope. When she attempted to surface, she was shelled by the escorting destroyers. She submerged to escape the artillery but was further damaged by a depth charge attack from the French destroyer "Bisson" while resting on the seabed. When "U-3" surfaced the following day, she was shelled and sunk by "Bisson". Fourteen of her crew were saved and captured, but seven died in the attack, including her commander, "Linienschiffsleutnant" Karel Strnad. "U-3" had no successes during the war.