Like all pre-H11British H-class submarines, H3 had a displacement of 364 long tons (370 t) at the surface and 434 long tons (441 t) while submerged.[2] It had a total length of 150 feet 3 inches (45.8 m), a beam of 15 feet 4 inches (4.67 m), and a draught of 12 feet (3.7 m).[3] It contained a diesel engines providing a total power of 480 horsepower (360 kW) and two electric motors each providing 320 horsepower (240 kW) power.[3] The use of its electric motors made the submarine travel at 11 knots (20 km/h; 13 mph). It would normally carry 16.4 long tons (16.7 t) of fuel and had a maximum capacity of 18 long tons (18 t).[4]
The submarine had a maximum surface speed of 13 knots (24 km/h; 15 mph) and a submerged speed of 11 knots (20 km/h; 13 mph). British H-class submarines had ranges of 1,600 nautical miles (3,000 km; 1,800 mi).[2]H3 was fitted with a 6 pounds (2.7 kg) Hotchkissquick-firing gun (6-pounder) and four 18 inches (460 mm) torpedo tubes. Its torpedo tubes were fitted to the bows and the submarine was loaded with eight 18 inches (460 mm) torpedoes.[2] It is a Holland 602 type submarine but was designed to meet Royal Navy specifications. Its complement was twenty-two crew members.[2]