The littoral submarines final design was settled 17 Mars 1939. A contract was signed with Kockums 20 September 1939 on construction of the first submarine U 1 of the series.
The Navy yard in Karlskrona was given the task of building submarines U 2 and U3. The U3 keel was laid 6 May 1941, launched 11 July 1942.
Trials crew was mustered 15 February 1943.
The boat was commissioned 1 November the same year. U3 was mainly operating as a school submarine for crews to the 18 submarines delivered during the war.
After WW II the submarines were put into depot were one crew alternatively manned one of 3-4 submarines. Technical innovations mainly with inspiration from German submarine technology, were installed when extending the lifetime of the boats. Snort mast, radar and radar warning equipment(IFF) etc. were tested and installed.
The littoral submarines or ”number submarines” as they often were called underwent a refit. In the U 3 case the refit was done by Kockums, August 1952 to April 1953.
U 3 was given a snort system and other improvements. Equipment that was left out was the after torpedo launch tube, AA gun, netsaw and the turbofan.
One of the two periscopes was substituted with the snort mast. The fin hydrodynamic shape was changed improving submerged speed from 7.5 to 9 knot.
A year later at refit 2, a new sonar was installed as was a new torpedo guidance system and a modern wireless.
Later the releasable keel was removed. Automatic weight control and one man steering was introduced and improved internal communications system. Electronic support antenna was installed on top of the snort head.
Six of the number submarines were during 1962-1964 converted to submarines named Abborren II (Perch) class. The whole aft part was substituted with a new design, one propeller with new main motor and a torpedo magazine for AU torpedoes in BT 3. All electronic systems were modernized. A new sonar was installed completely changing the forward part of the boat. The fin was also changed. These changes did not improve crew living standard.
