I find sinking a Battleship with three torpedoes very unrealistic. Now after saying that it did happen with as few as two.
The Kongo was the only Battleship sunk by a torpedo in the Pacific theater and was hit by two of three torpedoes fire by the USS Sealion.
From the Wiki:
On 20 November, they [
Yamato,
Nagato and the rest of the First Fleet] entered the
Formosa Strait. Shortly after midnight on 21 November, the submarine
USS Sealion made radar contact with the fleet at 44,000 yards. Maneuvering into position at 03:00,
Sealion fired three stern torpedoes at
Nagato and
Kongō. One minute later, two torpedoes were seen to hit
Kongō on the port side, while the third sank the destroyer
Urakaze with all hands. The torpedoes flooded two of
Kongō's boiler rooms, but she was still able to make 16 knots (18 mph). By 05:00, she had slowed to 11 kn (13 mph) and broken off from the rest of the fleet. At 05:20, she lost all power. Four minutes later, the blip indicating
Kongō on
Sealion's radar disappeared.
Kongō sank in 350 feet (110 m) of water with the loss of 1,200 of her crew, including the commander of the Third Battleship Division and her captain. She was the only Japanese battleship sunk by submarine in the Second World War, and the last battleship sunk by submarine in history.
Her sinking was only one out of three battleship sinkings in World War II caused by a submarine attack, the two others were the British
Revenge-class battleship
HMS Royal Oak and the
Queen Elizabeth-class battleship
HMS Barham.