The double-hulled submarine was just under 139 feet (42 m) long and displaced between 240 and 300 tonnes (260 and 330 short tons), depending on whether surfaced or submerged. The design of the submarine had poor diving qualities and several modifications to U-3's diving planes and fins occurred in her first years in the Austro-Hungarian Navy. Her armament, as built, consisted of two bow torpedo tubes with a supply of three torpedoes, but was supplemented with a deck gun in 1915.