HMS M3
 HMS M3 built by Armstrong Whitworth Newcastle Upon Tyne was an M class submarine of the Royal Navy.M3 was ordered from Armstrong Whitworth on 28 July 1916 and laid down at Elswick in December as an M-class submarine but was referred to as K20. She was launched on 19 October 1919 and commissioned as M3 on 16 March 1920. After completion of trials on 17 August she was placed in reserve.