Type U 3 was a class of u-boats built by the Imperial German Navy in the years before World War I. The two Type U 3 boats were ordered on 13 August 1907 and built at Kaiserliche Werft Danzig and commissioned during 1909. They had more powerful engines than the previous and were far more reliable. Neither boat saw active service during World War I, with both being used as training boats.
Type U 3s had an overall length of The boats' beam was , the draught was . The boats displaced when surfaced and when submerged.
Type U 3s were fitted with two Körting 8-cylinder two-stroke paraffin engines with a total of for use on the surface and two SSW double-acting electric motors with a total of for underwater use. These engines powered two shafts, which gave the boats a top surface speed of , and when submerged. Cruising range was at on the surface and at submerged. Constructional diving depth was .
The boats were armed with four torpedo tubes, two fitted in the bow and two in the stern. They carried six C/06 torpedoes instead of the older C/03 torpedo. C/06 torpedoes were slightly larger than the preceding C/03, they had a more powerful propulsion unit and could be fired under an angle. The boats' complement was 3 officers and 19 enlisted. Until the end of 1914, the Type U 3 U-boats could be equipped with a rotating Hotchkiss gun, and after 1915 U-3 was occasionally equipped with an extra 5 cm SK L/40 gun.
Both boats in the class were part of the training u-boat flotilla during World War I and did not see active service. Both were surrendered after the Armistice of 11 November 1918.