On the one hand there's Random House, which made it a habit of continually reprinting new editions of the series (in most cases, however, only the first eleven or so) solely to renew their copyright so that the rights would not revert to the estate of Robert Arthur (they didn't want to lose their royalties). At the same time, because so many changes were made in personnel, executives, and even owners over the years, very few at Random House knew precisely how valuable The Three Investigators was as a franchise, let alone why. This resulted in a number of ill-considered moves such as attempting to age the boys up, injecting more action into the stories, and creating new series runs which focused on being more modern and hip. Eventually, however, Random House did let the copyright lapse so that the books reverted to Elizabeth Arthur, Robert's daughter. This meant a) she could receive the royalties she had been rightfully owed due to her father's contract and b) she could, conceivably, find a new publisher and new authors who would do right by the series.
Technology Marches On: Unavoidable for a series written mostly in the 60's and 70's, but offenders which stand out are the constant references to payphones, the speakerphone Jupiter invents, walkie talkies and directional finders, the colored chalk to leave trails or send messages when cell phone texting could accomplish the same thing, and the Ghost-to-Ghost Hookup (which would likely not tie up all circuits today and could again be accomplished quicker and easier with texting). What is unfortunate is that Robert Arthur, the original author who came up with most of these inventions, took great pains to show his work and be current with technology, including that which the police and detectives would have; as usual the passage of time turned the series into an Unintentional Period Piece.