Every mystery story has a villain...usually. The Three Investigators always had at least one individual against whom they matched wits. Sometimes they weren't true villains so much as people who wanted secrecy (Stephen Terrill in "Terror Castle") or individuals who took some wrong steps but wanted to make up for it (Mr. Claudius in "Stuttering Parrot" and Professor Freeman in "Whispering Mummy"). Other times, the adversaries were truly dangerous criminals, and the Investigators had to outsmart the villains and solve the mystery.
Sometimes, the villains were revealed early in the stories, and the boys spent the entire case trying to outwit them. Other times, the villain was discovered only at the end after some evidence revealed that a previously trusted individual was the real culprit. In either case, the Three Investigators series brought out some of the best villains to be found in literature. They were usually motivated by greed, but sometimes had a personal score to settle with another character in the stories.
Below is a list of the more noteworthy villains that appeared in the Three Investigators series. I did not attempt to list all the villains or even the most sinister villains, just those that seemed the most interesting or unique for whatever reason. Of course, if you have others that you would like to suggest, you may e-mail me, and I'll consider adding them to the list.
You can read about the villains below by clicking on the appropriate hyperlinks.