The Three Investigators books have always been very popular in Germany. They are known there as Die drei ??? (Die drei Fragezeichen, meaning "The Three Question Marks"). Jupiter Jones was renamed as "Justus Jonas", a German adaption of his original name, while Pete Crenshaw is named "Peter Shaw". Bob Andrews retained his original name. The chauffeur's name is Morton.

While the American authors' novels in the series have been published there, German writers have added more, contributing about six new novels per year, with the count being 179 books during 2014. Taped radio dramas (Hörspiele) of the novels have been especially popular in Germany with most of them having been certified Gold or Platinum by the German Federal Association of Music Industry. In total, the radio dramas have sold more than 45 million copies and the books about 16 million copies in Germany (2013). A study conducted by the series' publisher Europa suggests that nowadays, most fans are between 20 and 40 years of age.

The radio actors, who have been narrating the plays since 1979, toured the country multiple times to perform plays in front of a live-audience. They broke their own Guinness World Record when performing Phonophobia – Symphony of Fear in front of 20,000 people at Berlin Waldbühne during 2014.

In the booklet of the German audio play "The Mystery of the Invisible Dog", the episode upon which it was based is credited erroneously to Nick West. Moreover, in Germany there are different revised editions of "The Mystery of the Scar-faced Beggar": one using Alfred Hitchcock as their patron, one using Alfred Hitchcock and Hector Sebastian, and another one using only Hector Sebastian (in Germany renamed as "Albert Hitfield").

New English-language Three Investigators titles were released during 2005 for the first time since 1990. The German 'American-English' series included the release of Poisoned E-Mail and The Curse of the Cell Phone. As of May 2008, a total of seven German stories have been translated and published in this format, and an eighth title was planned for publication during October 2008.