The Three Investigators book series ran from 1964 to 1987 and comprised of 43 books.
The sleuths in the series are Jupiter Jones, Pete Crenshaw and Bob Andrews.
The original book series started with the boys aged 13 to 14 years. Although the movie version picked boys of similar age they come across in the movie as somewhat younger especially the actor playing Bob who is noticeably smaller and who brings along his teddy bear on the adventure. It wouldn't be realistic to believe that a 13yr old boy would sleep with a Teddy bear whilst sharing a room with 2 friends his age. So it seemed a bit silly to add this to the movie.
As the second movie was released 2 years later it will be interesting to see if the boys fit into the roles better at that age.
In the book series Bob also has a leg brace he wore, due to multiple fractures inflicted when he rolled down a hill which wasn't done in the movie.
In the books the Three Investigators headquarters is a house trailer, hidden in a salvage yard owned by Uncle Titus Jones and Aunt Mathilda who Jupiter has lived with his the death of his parents in a car crash at the age of 4. It has many secret entrances, a telephone, small lab, a dark room and an office. The movie did the idea of secret entrances well in the movie and the headquarters was interesting to see.
Overall the movie enjoyable, was well acted for the age of the actors, filmed well and easy to watch.