Given the following context:

In 1943, a British naval officer from Canada, Commander Bolton and a few surviving crew members of his 50-man submarine Gauntlet swim ashore after unsuccessfully attacking German battleship Lindendorf. After a review, Captain Bolton is cleared of any wrongdoing and placed in charge of a small group of experimental X class submarines. Bolton is assigned by Vice-Admiral Redmayne to quickly train crews to man the submarines and sink the Lindendorf while it is hidden away in a Norwegian fiord.
Commander Bolton is to train three 4-man crews along the northern coast of Scotland for a trio of midget submarines equipped with side cargoes of explosives. He must overcome tensions with some of his former crew members, while keeping their activities hidden from outsiders and German airplanes. The crews successfully fend off an attack by German parachute commandos, who discover their base. Bolton is forced to make hasty preparations for his attack before their submarine base can be destroyed.
Two of the submarines are lost while attempting to cut through submarine nets at the entrance to the fiord. X-2, is sunk by a German E-boat's depth charges, and a second, the X-1, is scuttled. One submarine crew is captured and taken to the German battleship for interrogation. X-3, the surviving submarine penetrates the submarine nets in the fiord and places explosives under the German battleship. The submarine then manages to escape as the battleship explodes.

answer the following question:

What are the midget submarines named? 

X-3