SM U-3 or U-III was the lead boat of the U-3 class of submarines or U-boats built for and operated by the Austro-Hungarian Navy (German: Kaiserliche und Königliche Kriegsmarine or K.u.K.

The boat was commissioned into the Austro-Hungarian Navy in September 1909, and served as a training boat—sometimes making as many as ten cruises a month—through the beginning of the First World War in 1914.

On 12 August 1915, U-3 was damaged after an unsuccessful torpedo attack on an Italian armed merchant cruiser and, after she surfaced the next day, was sunk by a French destroyer.

U-3 was built as part of a plan by the Austro-Hungarian Navy to competitively evaluate foreign submarine designs from Simon Lake, Germaniawerft, and John Philip Holland.

[2] Two days later, while returning to Cattaro from the Straits of Otranto,[8] U-3 launched a torpedo attack on the Italian armed merchant cruiser Citta di Catania.

[2] 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.

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.