Thousands of users around the world have joined the M4 Project, using spare computing power to crack the codes.
The messages were encoded using the German Enigma machine, and outfoxed wartime experts at Bletchley Park.
Project leaders have already failed to crack the last remaining message, but insist it can be broken.
The three messages were unearthed by amateur historian Ralph Erskine, who submitted them to a cryptology journal in 1995 as a challenge for codebreakers.
They were sent in 1942, during a period when the Allies were unable to crack German codes because of the introduction of a new code book and a more complex version of the Enigma machine.
SOLVED CIPHER #2
Found nothing on convoy’s course 55°, [I am] moving to the ordered [naval] square. Position naval square AJ 3995. [wind] south-east [force] 4, sea [state] 3, 10/10 cloudy, [barometer] 28 mb [and] rising, fog, visibility 1 nautical mile
Stephan Krah, a German enthusiast, wrote the M4 Project software - named after the M4 Enigma machine used to encode the messages - in an effort to unravel the codes’ mystery.
The first code was cracked on 20 February, and was confirmed as a message from the commander of a German U-boat, Kapitanleutenant Hartwig Looks.
The second resolved code was less dramatic than the first, which detailed the aftermath of a clash with an Allied vessel.
The newly-deciphered code is little more than a status report and a confirmation of position