The Three Investigators was a juvenile detective book series written by Robert Arthur Jr., originally called "Alfred Hitchcock and the Three Investigators". It centered on a trio of high school boys who live in the fictional town of Rocky Beach, California. They are:Jupiter Jones, First Investigator. Head of the firm and known for his remarkable powers of observation and deduction, he is stocky, muscular, and a bit roly-poly. He has a round face which often looks stupid but which hides a sharp intelligence. Jupiter has an excellent mind, and he is rather proud of it. He has many good features, but undue modesty is not one of them.Pete Crenshaw, Second Investigator. Tall and muscular, sturdy and courageous, he excels at athletics. Inclined to nervousness before anything happens, but a tower of strength in any kind of trouble. He is Jupiter's right-hand man when it comes to trailing suspects and other dangerous activities. Pete's father is a special-effects man who works at one of the movie studios in Hollywood.Bob Andrews, Records and Research. Slight of build, small but wiry. Studious in nature, he is something of a scholarly type with an adventurous spirit. He has great nerve and the courage of a lion. Adept at research, he works part-time at the local library which enables him to hunt up information needed for their investigations. Bob's father is a feature writer for a big newspaper in Los Angeles.The boys spend their free time solving various mysteries rather than true crimes, mysteries which tended to be far more bizarre, unusual, complex, and intriguing than those of The Hardy Boys and other Kid Detective books of the day, and with protagonists who were simply ordinary, middle-class American boys, without the riches or special advantages of sleuths such as, again, The Hardy Boys or Nancy Drew, both of which had famous fathers who helped them out in their cases a great deal.The three boys make an excellent team. Having formed the firm of The Three Investigators, they use their spare time to solve any riddles, enigmas and mysteries that come their way. Their motto is "We Investigate Anything". Headquarters for The Three Investigators is a damaged 30-foot mobile home trailer within the salvage yard run by Jupiter's Uncle Titus and Aunt Mathilda which has been cleverly hidden from view by stacks of junk which surround it. It is accessible only by several secret passages and hidden entrances including their favorite, Tunnel Two. Headquarters contains a small laboratory, a dark room, and an office with a desk, typewriter, telephone, tape recorder and reference books. All of their equipment was rebuilt from junk that came into the salvage yard.For traveling long distances, the boys have the use of a gold-plated Rolls Royce, complete with a chauffeur, Worthington. Jupiter won the use of this auto, for thirty days, in a contest. (A grateful client from the seventh book in the series, The Mystery of the Fiery Eye, indefinitely extended the time that they could make use of the Rolls.) For local travel, the boys ride their bicycles or have one of the salvage yard helpers, Hans or Konrad, drive them in one of the trucks.Adding to this quasi-realism was the real-life movie director, Alfred Hitchcock, who appeared in the original texts of the first thirty titles. His character provided the introductory and closing remarks in each book and, acting as a mentor, he was occasionally called upon by The Three Investigators during the course of solving a mystery. The real Alfred Hitchcock had little to do with the creation of these books. He was simply paid a handsome percentage for the use of his name and character. This provided brand-name recognition and helped boost sales of the books.Following Robert Arthur's death, the writing of the series was taken over by several successive authors—two titles by Nick West (pseudonym of Kin Platt), a few by Marc Brandel, and the bulk of them penned by William Arden (pseudonym of Dennis Lynds) and M.V. Carey.The long-standing popularity of the series in Germany has resulted in two live-action movies, The Three Investigators and the Secret of Skeleton Island and The Three Investigators and the Secret of Terror Castle.
The Three Investigators provides examples of:
Absurdly Spacious Sewer: The Mystery of the Silver Spider had the young heroes escape the villains in the storm drain system of a very old city. They meet up with some allies rowing a boat through the drain.
Added Alliterative Appeal: A favorite of the book titles, as in Green Ghost, Silver Spider, Crooked Cat, Flaming Footprints, Singing Serpent, Monster Mountain, Dancing Devil, Headless Horse, Deadly Double, Sinister Scarecrow, Purple Pirate, Missing Mermaid, Trail of Terror, Rogues' Reunion, Creep-Show Crooks, Wrecker's Rock, Cranky Collector, Dancing Dinosaur, House of Horrors and Savage Statue. By the early 80s, alliterative titles were almost obligatory, before being dropped for the Crimebusters relaunch.
Also, Jupiter Jones, Java Jim, and Anti-Villain Claude Claudius.
Aesop Amnesia: Several times (for example, in Shrinking House and Dead Man's Riddle) Skinny Norris receives a bad scare/brush with danger and death that seems to convince him of the error of his ways. But in the next book he's always back at it again, jealously pursuing and competing with the Investigators, until finally being Put on a Bus (and maybe having a Heel-Face Turn) in Headless Horse.
All That Glitters: The novels contained versions of this periodically. One that comes to mind lacked a clear moral: a sunken riverboat holding a watertight chest contained millions of dollars—in worthless Confederate money. It may have been worthless when the book was written, anyway. But these days, preserved Confederate money is worth more than US currency of the same denomination, with mint-condition bills of $100 and $500 worth tens of thousands of dollars. The real irony, of course, is that Confederate money is so valuable now due to most of it being destroyed precisely because it was considered worthless.
Anti-Villain: A number of the boys' enemies turn out to be this, either having sympathetic reasons for doing what they do, not genuinely wishing to hurt anyone, being Forced Into Evil, performing a Heel-Face Turn, or simply being too Affably Evil to be hated. Good examples are Mr. Claudius of Stuttering Parrot, Professor Freeman of Whispering Mummy, Arthur Shelby of Coughing Dragon, Professor Shay of Phantom Lake, Senor Santora of Haunted Mirror, and Mrs. Chumley of Sinister Scarecrow.
Arch-Enemy: That perennial ne'er-do-well Skinny Norris. Later, when Skinny Norris had a Heel-Face Turn, the position was taken by the mastermind Victor Hugenay.
Artist Avatar: Sort of. When the characters of Bentley and Allie were introduced in Singing Serpent, the illustrator based the looks of the latter off his own daughter and the former off of himself.
Author Appeal: M.V. Carey, a later author in the series, seems to have had a fixation on the supernatural since it figures prominently in at least four of the titles she wrote: Singing Serpent, Invisible Dog, Haunted Mirror, and Magic Circle. She also wrote stories involving cryptozoology and UFO sightings (Monster Mountain and Blazing Cliffs, respectively). Even Flaming Footprints and Sinister Scarecrow had vaguely supernatural elements, although the "ghost" leaving footprints and walking scarecrow were just done with chemicals and a costume.
Author Avatar: Word of God reveals that Robert Arthur saw much of himself in Bob Andrews and often wrote his traits into the character. Bob was also more often the POV character in Arthur's books than Pete and Jupe were.
Bank Robbery: The books which aren't looking for Buried Treasure, lost masterpieces, or solving puzzles/riddles are usually this, or involve looking for the loot from one that had been hidden. Examples of actual robberies: Vanishing Treasure (which actually takes up so much of the narrative it's practically a Heist Book), Coughing Dragon, Sinister Scarecrow (variation—a museum robbery); finding the loot: Skeleton Island, Talking Skull, Crooked Cat, Death Trap Mine.
Battle Butler: Worthington, on occasion. He certainly acts as this in the very first book, even abandoning the Rolls-Royce to help Bob rescue Jupe and Pete inside Terror Castle.
Usually averted otherwise throughout the series, in fact as often as not it is defied by Jupiter who often suspects or at least does not discount servants and other seemingly irrelevant characters. At least once, though, Jupiter did almost fall prey to the trope when he continually discounted the possibility that Mrs. Chumley could be the scarecrow, and he also overlooked Doc Dawson in Nervous Lion until almost the last minute when a Contrived Coincidence allowed him to discover the smugglers' coded message in his medical bag.
Bilingual Bonus: Appears on occasion, such as Salsipuedes Street in Dead Man's Riddle meaning "get out if you can", something which a resident of many California towns (or native Spanish speakers) would get right away.
Blackmail: The source of the plot in Haunted Mirror—Con Man Diego Manolos was able to use the Chiavo glass to perform various robberies and crimes, culminating in framing the future president of Ruffino for thievery. He then kept evidence of this (photographs and news clippings) which he used to threaten the president into giving him money, a fancy house, and a high advisory position in the government, lest he lose the next election in favor of a true dictator who would cause a terrible uprising and much suffering in the republic. Obtaining this evidence (hidden on the mirror) is what lies behind the various attempts by both Gomez and Santora to get the mirror, including the Scooby-Doo Hoax.
Bluffing the Criminal: Jupiter has done this on several occasions to receive an incriminating confession.
Bound and Gagged: Lots of examples as it's a staple of the genre, including in the very first book. Particularly memorable examples occur in Vanishing Treasure and Fiery Eye.
British English: The puzzle of Dead Man's Riddle is written entirely in Cockney rhyming slang.
Buried Treasure: Some of the more memorable entries involve these—Stuttering Parrot, Skeleton Island, Fiery Eye, Laughing Shadow, Phantom Lake. The last one was even Pirate Booty.
Character Tic: Jupiter is known for pinching his lower lip when he is thinking furiously and his brain is about to crack a case.
Chekhov's Gun: Many across all the books, a lot of them introduced in casual, one-line references very easy to miss upon first reading. Enough to be a Chekhov's Armory at times.
Circus Brat: Andy of Crooked Cat, to his grandmother's chagrin and extreme disapproval. This changes by the end of the story when his value to the carnival and enjoyment of the life there is proven to her.
Circus of Fear: The carnival in Crooked Cat averts this trope since, other than the numerous accidents plaguing it that make the performers think it's bad luck, the show is quite harmless and enjoyable. The abandoned amusement park next door with its rickety roller coaster, ominous fun house (with a Hall of Mirrors, natch), and murky tunnel of love, however...
Clear Their Name: A big part of the plot of Screaming Clock is this for Harry's dad; also plays a smaller role with Pico in Headless Horse.
Cloud Cuckoolander: A number of these appear throughout the series whether as clients, witnesses, or clue bearers. Irma Wagoner from Stuttering Parrot (who is almost a bird-owning version of the Crazy Cat Lady), Miss Agawam from Vanishing Treasure, and Mrs. Darnley from Haunted Mirror are prime examples.
Cold-Blooded Torture: A few of the villains threaten the boys with this while holding them Bound and Gagged, but the most memorable would be Duke Stefan in Silver Spider who actually holds them in a medieval dungeon with all sorts of torture implements and threatens to put them in the Iron Maiden.
Color-Coded Characters: The three boys each have their own color of chalk to use for marking trails, identifying hideouts or places of importance, or leading searchers when they need help. Jupe's is white, Pete's is blue, and Bob's is green. (Overseas, at least one German reprinting has changed this, however, to be red, white, and blue.)
The reason for the change in Germany is the cover design. Among other things Aiga Rasch designed a label for the series, with white, red and blue questionmarks. It is an on-going joke between German fans that the order of the colour is wrong, because the colours used in the German books are actually white (Justus/Jupiter), blue (Peter) and red (Bob).
The Commissioner Gordon: Chief Reynolds, from Green Ghost onwards. Usually he is the helpful and friendly sort, albeit at times a bit exasperated with the boys' tendency to get into trouble and Jupiter's arrogance, but sometimes (mostly under M.V. Carey) he falls into outright Teeth-Clenched Teamwork; whether this was due to resenting all the cases they solved (thus showing up the Rocky Beach police force's weaknesses and need to depend on them for help) or, again, just because of Jupiter's nosy and cocky nature, was never explained.
Concealing Canvas: In The Case of the Weeping Coffin, the eccentric millionaire's house is littered with these, to the point that the curtain that doesn't conceal a safe is interesting.
Con Man: A popular villain type. Appears in Laughing Shadow, Singing Serpent, Monster Mountain, Death Trap Mine, Magic Circle, and Sinister Scarecrow.
Conviction by Contradiction: Happens a few times when Jupiter is unmasking the criminal. However unlike with Encyclopedia Brown, usually said clue actually does prove there was no way the suspect could have said or done what they claimed and they had no reason to lie or be mistaken. They also almost always confessed, and in several cases this slip-up allowed the boys to find actual hard evidence confirming their guilt. Examples: Marechal and the Countess in Shrinking House, Shay in Phantom Lake, Jefferson Long in Magic Circle, Mrs. Chumley in Sinister Scarecrow.
Cool Car: The Rolls Royce Jupiter wins for 30 days in the first book. Even comes with its own driver, Worthington.
Couldn't Find a Pen: The Yaquali boy who threw the statuette with a message for help inside in Laughing Shadow had to use his own blood to write it. In Headless Horse, the boys at first guess Don Sebastian Alvaro did the same thing while dying in his hidden cave, but instead he used black paint he had with him. This is plot-significant.
Cover Identity Anomaly: The "jolly fisherman" Mr. Farrier from Flaming Footprints falls under suspicion due to this—possessing brand-new equipment and pristine, expensive new clothing...but having a dirty, dusty old car that doesn't at all match the rest of him. He turns out to be a jewel thief in disguise, who had bought all the new, expensive things with a stolen credit card.
Covers Always Lie: Happens from time to time. One example is The Mystery of the Invisible Dog, the cover of which shows the investigators cornered by a large transparent feral dog. The invisible dog in the story? A small glass statue, which they are hired to find. Although said feral dog is a reference to the Carpathian Hound of legend, which the statue depicts.
Covers Always Spoil: At the same time, a few of the covers of the books also spoil major plot points or even endings. Vanishing Treasure depicts the boys with the belt in Headquarters, revealing they get it back; Fiery Eye shows them digging the eponymous jewel up, thus completely deflating the tension of the search for the busts; Silver Spider shows the eponymous spider in a web; one version of Laughing Shadow shows Indian Head Mountain, thus hinting at the meaning of "eye of the sky" before it is revealed; Coughing Dragon makes it fairly clear that said dragon is a robot; and a very eagle-eyed observer will notice the awning patch with the stripes going the wrong way on the eponymous Shrinking House.
Creepy Cemetery: Classic example in the Merita Valley graveyard of Stuttering Parrot where John Silver hid the stolen painting. Not only is the whole scene in the graveyard tense, suspenseful, and eloquently described, lingering in the reader's mind for a long time afterward, but it was so memorable that illustrations of the scene became the endpapers for the hardcover editions for most of the series' run.
Cult: Of the eponymous Singing Serpent; despite (or perhaps even because of) being a Scam Religion, it's actually explored fairly seriously and sympathetically for the victim of it.
Deadpan Snarker: At various times all the boys get in on this, but Bob especially seems prone to it in the M.V. Carey books, often of a Genre Savvy nature. (And one time where he actually seems Genre Blind, it's Jupiter who calls him out on it.)
The earliest books by the series creator Robert J. Arthur made a strong attempt to differentiate the boys' personalities, as well as showing their different interests and intellect levels, so that depending on the case Bob or Pete might have the clue to crack the case rather than Jupe, and their temperaments and personalities determined what roles they played in the investigation or finding the solution. Later books in the series, however, tended to lose some of this characterization so that, as one fansite put it, it almost didn't matter whether Bob or Pete were in the scene or said a certain line because they had become interchangeable. Who made the most jokes or was the Deadpan Snarker also varied from author to author (and book to book).
Jupe's being a Former Child Star factored into a great many of the early cases (unsurprisingly, as they were written by the author who invented that Backstory), and the subsequent authors tried to include it in their early books too...but eventually this element faded from the series, save for one late attempt by Marc Brandel and another in the Crimebusters series to bring it back.
Jupe's Teen Genius status also fluctuated, even within the Robert Arthur books, where things one would think he'd have discovered and read about turn out to be something he's never heard of—his error in not recognizing Octavian to be Caesar Augustus is particularly egregious—and things he learned in one case don't carry over to another—discovering that uncut diamonds look just like regular rocks when they come out of the earth in Moaning Cave somehow escapes his memory when the coded cable message in Nervous Lion refers to "rocks".
And Pete, who somehow has knowledge on how to work with trained animals in Crooked Cat courtesy of his father (how would a movie producer come by such knowledge?), shows no such skill in Nervous Lion when confronted by George; this is particularly noteworthy since he otherwise does show knowledge he learned from his father when talking about Jay Eastland and watching the filming.
What is Jupe's Character Tic for when he's in deep thought? In Arthur's books, it's pinching his lip. In other writers' books, it's chewing his lip.
Dying Clue: A few of the riddles or searches for lost treasure involve one of these. The Chumash Hoard's hiding place is revealed by the Famous Last Words of Chief Magnus Verde, while the delirious Joshua Cameron of Shrinking House babbled a message for Marechal about the hiding place of the lost Fortunard. Don Sebastian Alvaro of Headless Horse, dying in a cave after the American deserters hunted him down, left one on the wall next to him that was particularly cryptic, but inspired: "Ashes Dust Rain Ocean", meaning that like everything else in life, the Cortes Sword had gone back to where it began...
Easy Amnesia: Bob suffers from this in Silver Spider, thanks to a Tap on the Head causing him to conveniently forget where he hid the spider. Interestingly, when he suffers a second blow to the head later, although he doesn't display the proper side effects from this, he doesn't suddenly get his memory back, either.
Everybody Did It: Or at least, everyone except Letitia and Dr. Woolley in Sinister Scarecrow. Notable in that while Mrs. Chumley and the Burroughs were working together to rob the Mosby Museum (at least, eventually), Gerhart Malz's forgery plan was completely separate and had nothing to do with the scarecrow. As Hitchcock himself says, "Rarely did the boys have so many suspects turn out to be guilty!"
Another example occurred two books earlier in Magic Circle: Harold Thomas/Charles Goodfellow turned out to be responsible for both the theft of the manuscript and the films; Marvin Gray was his partner in conning Madeline and gaining the manuscript, while Jefferson Long was the mastermind in ransoming the films. This one is justified in-story by the fact that all three criminals were acquainted before, having been part of Madeline's coterie of actors and film studio workers, as well as her witches' circle.
Evil All Along: The Countess and Marechal from Shrinking House, Professor Shay from Phantom Lake, "Thurgood" from Death Trap Mine, Doc Dawson from Nervous Lion, Professor Walsh from Moaning Cave...
The prize Jupiter won was to have a Rolls Royce at his disposal for 30 days. When one month was nearly over, Jupiter argued that 30 days actually amount to 720 hours of service.
The relevant exact words were "30 days of 24 hours each (720 hours), Jupiter argues that by that wording only full days count, and they've only used 3 of them.
Many riddles and puzzles in the series rely on these, but one of the best is Laughing Shadow: the Chumash chief whose Famous Last Words tell the location of the hoard said "it is in the eye of the sky where no man can find it". It's hidden literally in an "eye of the sky", a cave inside a high mountain shaped like an Indian's head, with the cave inside the eye...and it is small enough no man can enter it, but a child or young teen can.
Excited Chapter Title!: Every book in the series has at least one chapter title like this, usually more. Invariably it is "Trapped!", "Captured!", or some variation of this where the boys are in some sort of danger.
Based on the names of the characters involved (Kaluk, Demetrieff, Kerenov) and the coup which took place in the Backstory, the plot of Flaming Footprints reads like a search for the lost crown jewels of Imperial Russia (or a Ruritania parallel); note the double-headed eagle symbol of Lapathia was originally used by the Byzantine Empire and then later both Austria-Hungary and Russia.
A strong case could be made that Jim Hall, Jungle Land, and George of Nervous Lion are either this or a Homage to Ralph Helfer, Africa U.S.A., and Zamba.
The history of the Fiery Eye dovetails in many ways with that of the Hope Diamond, plus a bit from the famous stolen rubies that jewel thief Klaus Gudden hid in a clay cat statue in 1894.
Also happens a few times with a hidden treasure when the riddle/puzzlemaker is particularly clever (or trollish). Fiery Eye and Dead Man's Riddle are the most notable. (The Fiery Eye example also involves Jupiter, again, giving the fake version to the villains after having been fooled himself earlier.)
Folk Hero: El Diablo of Moaning Cave. Explicitly compared to Robin Hood in-story, he was a young Spanish nobleman whose family lands had been stolen, lost, or given away thanks to the influx of Californian settlers and, seeing the Americans as usurpers and thieves, turned into an outlaw to rob the government and drive them out. Though the Spanish peasantry saw him as a defender of justice and righter of wrongs, he was caught, arrested, tried, and about to be executed until freed by some of his friends, allowing him to escape (though badly wounded) to his cave hideout. There he was chased and surrounded by the authorities but never emerged, because he believed it was Better to Die than Be Killed. He even became a literal King in the Mountain, as after his death the legends persisted that he was still alive, whether hiding in the cave or having escaped by some secret means, and would return one day to help his people when the need was greatest.
Former Child Star: Jupiter as a very young child, known as "Baby Fatso". While this generally gives him knowledge of show business and acting that comes in handy in various cases, it particularly allows him to be useful in Crooked Cat, since it extends to knowing much about circus and carnival life. He even gets to perform as a clown, albeit out of desperation, during the climax. The show he was once a part of also gets featured in a couple of cases, The Mystery of the Rogues' Reunion and one in the Crimebusters series.
Frameup: Happens fairly often, such as: Harry's father in Screaming Clock, Hank Morton in Nervous Lion, Stebbins in Phantom Lake, Pico in Headless Horse.
Gaslighting: While the intent was only to distract her or force her to leave the estate so the museum robbery could go off as planned, the exploitation of Letitia Radford's fear of scarecrows and bugs in Sinister Scarecrow is malicious enough to count as this trope, and she nearly does go mad. Ends up being subverted, however, when the villains' concern that the boys will catch on to their scheme leads to the scarecrow attacking the boys, thus proving it isn't just in Letitia's head.
Game Between Heirs: Variation. The eponymous Dead Man's Riddle does allow a competition between Dingo Towne's family members to find his hidden jewels, but because he hated most of them and distrusted or misjudged the rest, he opened the game to any random person in town who could solve it.
Ghost Town: The former mining town Powder Gulch is featured fairly prominently in Phantom Lake as a place visited by Angus Gunn when preparing his wife's surprise (and treasure-hiding place). Another, Hambone, appears briefly (but still as a plot point) in Death Trap Mine.
Glass-Shattering Sound: The purpose of the screaming clock, as inspired by the trick professional screamer Bert Clock used to like doing for his friends.
Good Detectives, Good Clients: Played utterly straight for almost every book in the series, with the boys' clients either being innocent victims of the con men/robbers/kidnappers, or bystanders caught up in such schemes by being in the wrong place at the wrong time or stumbling upon an important Plot Coupon. Which is why the subversion in Shrinking House where both the elegant Marechal and the beautiful Countess turn out to be the swindling bad guys, the seemingly villainous DeGroot is actually a Dutch cop in pursuit of them, and Joshua Cameron himself was a master forger is so shocking and one of the most memorable entries in the whole series. Subverted again in Dancing Devil but with less fanfare.
Good Scars, Evil Scars: Played with for the Whisperer—although his scar is ugly, vicious-looking, and disturbing, he himself is as good-hearted and friendly as can be. On the other hand, the whole reason Stephen Terrill had him as his manager was because he looked sinister, making people afraid to go against him and thus leave Terrill in peace/give him what he wanted. Subverted in the end since the Whisperer is Terrill and the scar itself is fake.
Narrative Profanity Filter: The mynah bird Blackbeard from Stuttering Parrot was taught a number of racy pirate slang and swear words "not meant for decent company" according to the old lady who bought him. When the boys first get to hear him speak, the text notes he "burst into a string of expressions the boys knew their families would never approve of".
Graceful Loser: Several of the baddies, but Mr. Won (who returns the deed to Verdant Valley despite the Ghost Pearls being destroyed) and Hugenay in Stuttering Parrot stand out. The latter even calls the boys to congratulate them and tip them off to having gained the treasure.
Vanishing Treasure: A case about a stolen belt, and then one about a lady being haunted by gnomes, turns into a bank robbery case.
Monster Mountain: A case about a missing key turns into a sasquatch hunt, then a race to rescue a kidnapped woman who'd been replaced by an imposter.
Dancing Devil: A case about a bunch of missing black cases becomes one about the missing statue of the title.
Sinister Scarecrow: A case about helping a traumatized and paranoid woman afraid of ants turns into preventing a museum robbery.
Handicapped Badass: For the first three books of the series, Bob wears a leg brace due to multiple fractures he suffered after falling down a steep hillside while climbing. The trope is played with in a number of ways, however—while Bob is unquestionably courageous and does not let his injuries keep him from investigating or being of use as many ways as he can, he isn't shown to be badass, not being capable of anything more than the other boys, or any normal person. At the same time he's not Inspirationally Disadvantaged either, nor does anyone ever assume that Disabled Means Helpless; the only trope he comes close to fulfilling is Genius Cripple due to his intellect, but while he is smart, the real genius of the group is Jupiter. So in the end he manages to be just what he appears to be on the surface—a normal boy who got injured, was temporarily crippled, then healed and became just like his detective partners, a very realistic way of addressing disabilities that is quite refreshing for the time the stories were written. And the fact his injuries were the sort which could heal easily over time suggests it wasn't a case of Robert Arthur initially intending to hew to this trope (or Inspirationally Disadvantaged) and then backpedaling when he realized the Unfortunate Implications—he always intended Bob to get better.
Have a Gay Old Time: As usual for books written during that era, the word "queer" tends to turn up a lot. There's also an unusual word usage in Crooked Cat where Andy, during his barker spiel, calls Pete a "nimrod". Considering the fact he was about to compete in a shooting gallery, comparing him to a great historical general makes sense, but thanks to Bugs Bunny no one in modern times can take that word seriously any more...and so would likely conclude Andy was mocking Pete. Comparing the publication date of the book to those of Looney Tunes episodes, it's remotely possible this more modern meaning was actually known at the time, and so its usage there intentionally had a double meaning.
Hidden in Plain Sight: A favorite for solving various mysteries. The silver spider turns out to be in a spider web with a real spider, the crooked cat was in the work basket to be repaired, the diamonds in Nervous Lion were in the bars of the cages they'd had with them all along, the Phantom Lake turned out to be the man-made "view down the loch" where the treasure was buried, the invisible dog (a crystal statue) was hidden in a swimming pool, and the Cortes Sword of Headless Horse turned out to be painted and nailed to the side of the Cortes statue.
History Repeats: Happens to Marvin Gray of Magic Circle, who gets in a car accident exactly the same way Madeline Bainbridge's boyfriend did thirty years before. Unlike him, he survives.
I Don't Like the Sound of That Place: The Three Investigators seem to keep ending up at places like this: Terror Castle, Skeleton Island, Phantom Lake, Monster Mountain, Death Trap Mine, Shark Reef, Wrecker's Rock...
Identical Stranger: Jupiter has one in Deadly Double. Hans and Konrad's cousin Anna also has one, leading to Imprison and Replace by her Criminal Doppelgänger so that her Con Man husband can steal Anna's money. Foreshadowed by Anna's odd refusal to reply in German when Hans and Konrad speak it to her, although she clearly can understand them. The Identical Stranger is native to Germany but knows an entirely different dialect, so it would be painfully obvious she wasn't actually their cousin.
Impersonating an Officer: In Screaming Clock Hugenay the Classy Cat-Burglar has one of his men dress up as a police officer. When the cops show up and try to use that as a charge, he points out the the fake cop is in fact wearing a New York Police uniform (the series is set in California), and as such cannot be accused of impersonating the local police.
Imposter Forgot One Detail: The con man pretending to be Wesley Thurgood in Death Trap Mine forgot/didn't know the real Thurgood didn't have blue eyes.
One case involving a whale where a suspect accidentally blurted out its species.
On another occasion (in Laughing Shadow), Ted Sandow asks what the "???" on their business card means. This is a Once an Episode thing which wouldn't normally be significant, but Jupe notices that he didn't actually read the card, and must have seen it before. Of course this is just a Red Herring, as the true villain of the story is Mr. Harris, not Ted.
Jefferson Long of Magic Circle gave himself away by reporting the exact number of assailants who stole the films before that information was released to the press.
Senor Santora of Haunted Mirror, when told that a robber had tried to take the Chiavo glass, referred to the robber as a "small man" despite supposedly not having seen him, thus suggesting he did indeed know of the thief. Since any person would seem small next to the mirror, however, this only serves as circumstantial evidence and the boys must track down further clues to determine his guilt or innocence.
It Only Works Once: Variation—in Shrinking House the boys are told that people unfamiliar to the area where the case takes place only fall in the barranca on the property once (thus learning it's there and avoiding it in future). This is then used to identify which character is the masked thief (who falls in right near the start of the book) based on whether they fell into the barranca later.
It Won't Turn Off: In Terror Castle, the boys are told about the cursed Ominous Pipe Organ that played both when nobody was near it, and when it was unplugged. (Stephen Terrill supposedly did experiments to be sure.) They later investigate the organ, but it turns out to be just a trick, like everything else in the Haunted House.
I Was Just Joking: Happens a few times when Bob or Pete makes a sarcastic or humorous suggestion, only for Jupiter to have a Eureka Moment thanks to taking it somewhat seriously. Example from Headless Horse, when they're trying to find Condor Castle:
Pete: Maybe we need an old Indian map, and an old Indian to read it for us.
Jupiter: Pete, I think you've got it!
Pete: Gosh, First, it wasn't that bad a joke.
Karma Houdini: Generally the series is not known for these—either the villain gets sent to jail at the end of the story, or if it's about pursuing a lost fortune/item, they fail to get it and leave at the end—otherwise unpunished for any fell deeds they committed while on the search, but failing to find what they coveted would be punishment enough in their eyes. At least twice though, a villain did actually escape despite the very real crimes they committed—Jensen, Mr. Won's minion in Green Ghost, gets away (as does, for that matter, Mr. Won), while Rawley and his gang escape in Vanishing Treasure. This last is notable not only because of the determined attempt by the boys that the "gnomes" of that case did not get to be this trope, but because of what Rawley did: he was an exceedingly clever criminal who almost got away with his loot, he coldly threatened Jupe and Pete with being dumped off a ship with bags of silver coins tied to them, and while he, like so many kids in the neighborhood growing up, had been one of Miss Agawam's gnomes and came to visit her frequently, he repays her as an adult by using the "gnomes" to frighten her into moving so she wouldn't discover the tunnel being dug under her house. Even though the "gnomes" do get caught, and Miss Agawam moves to a new home with no harm done, that's still a nasty thing to do to such a sweet, harmless person...and yet he still gets away.
Kick the Dog: While a number of villains do terrible things (for a kids' series version of terrible—as usual, none of them ever commit murder that we know of), two which stand out would be the villain of Laughing Shadow who indulges in child slave labor to find the treasure (and has every intention of either killing them or leaving them to die after he's gained the treasure so as to cover his tracks) and Mrs. Chumley of Sinister Scarecrow who uses her knowledge of Letitia Radford to create the terrorizing scarecrow; this last is one of the few things keeping her from being a completely sympathetic villain.
La Résistance: The Minstrel Party of Varania, opposing Duke Stefan in Silver Spider.
Large Ham: No pun intended (remember that Berserk Button), but it's pointed out several times how much Jupiter has a flair for the dramatic and loves hamming it up, usually when unmasking a criminal, solving a riddle, or revealing where the treasure they're looking for is hidden.
The Leader: Jupiter Jones is a Type I example, since he most certainly is a mastermind, assembled his friends for the specific purpose of becoming investigators, is always the one with goals for a case and coming up with the boys' plans, is even more of The Smart Guy than Bob, and is often a Guile Hero. He's also a Type IV, being the most charismatic of the three and the one who (other than a few examples of grumbling and mutiny, usually from the Cowardly Lion Pete) is always listened to and agreed with. He appoints himself First Investigator and makes no effort to hide why he considered this the right choice, demonstrating time and again what skills place him in the position. At the same time, despite his pomposity, he always gives Bob and Pete credit where it is due, working to include their thoughts and reasoning in cases, never belittling them or their skills (at worst getting annoyed when they don't see an "obvious" clue that he does, or gently mocking Pete for his cowardly ways). He makes an effort on several occasions to point out to clients how all three of them are essential and have important roles, times when only they can do what needs to be done, and he also encourages them to be more proactive in pursuing clues and thinking deductively even as he remains in charge.
Linked List Clue Methodology: Twice. The first time is played with—in Stuttering Parrot the messages of the seven birds all lead generally to one place where the treasure is hidden, but each message after the ones which lead them there are part of a linked chain to help them find the exact hiding place. Played straight with the "Dead Man's Riddle", although if Jupiter had recognized a stealth clue in the reference to a "posh Queen" he could have taken a shortcut and jumped straight to the end of the riddle.
Locked Room Mystery: A near-literal example occurs as a subplot in Shrinking House, where the boys have to figure out how someone has been getting in and out of an impregnable concrete art studio with only one locked door in order to move Joshua Cameron's paintings. Turns out there is another entrance that was overlooked—a hole high in the back wall holding an exhaust fan.
Look Behind You: Roger Callow of Dead Man's Riddle actually falls for the subverted version of this trope—being sure that the boys were simply trying to make him turn around so they could get away, when in actuality Billy really was behind him and could run off to warn the police.
MacGuffin: While usually the boys are pursuing important clues/items, or even the actual treasure, just as often it's an item with little or no purpose (the Silver Spider of Varania is needed to crown its king and thus needed to prevent the Regent for Life plot of the Evil Chancellor, but otherwise does nothing), it's only a clue or a hiding place for one that leads somewhere else (the crooked cat, the haunted mirror, the paintings from Shrinking House) and thus becomes irrelevant once it has served its purpose, or it's a Red Herring. Appropriate that this would appear, considering who the series is named after.
MacGuffin Delivery Service: In most of the stories with riddles leading to lost treasures, the villains sit back and wait for the boys to solve it for them, then swoop in to take it from them. Sometimes the villains are figuring out the riddle too and thus happen to arrive at a locationat the same time as the boys (justified in Linked List Clue Methodology cases—because the clues have to be figured out and followed in order, the heroes and villains meeting up is bound to happen eventually) and then take it away from them, but usually they merely follow the boys and let them do the work. Classic example: the Percivals from Dead Man's Riddle.
MacGuffin Melee: Dead Man's Riddle is a particularly striking example, with the Investigators, the Percivals, Skinny Norris, andRoger Callow all looking for the treasure.
Magnetic Plot Device: Also happens a lot, usually with whatever item they're seeking or the clue which will solve the mystery/find the treasure, but the crooked cat and the paintings from Shrinking House take the cake.
Manipulative Bastard: Mr. Harris of Laughing Shadow. Not only is he a Con Man who is using and misleading Miss Sandow in order to get the Chumash Hoard (and is using child slave labor to find it), he manages to fool Jupiter into thinking he's a good guy and encourages his false assumptions about Ted Sandow being the villain so that he can kidnap Bob and Pete, then nab the treasure. He would have gotten away with it if it hadn't been for his pet kookaburra and the telltale evidence proving he wasn't actually a vegetarian.
Also, Laslo Schmidt of Moaning Cave and the Amazing Gabbo of Crooked Cat.
Maybe Magic, Maybe Mundane: It is never explained how the old gypsy king, Anton, has the knowledge he does in Silver Spider—after using an herb of some sort as a truth serum and being unable to learn where the missing spider is thanks to Bob's lost memory, he makes two odd pronouncements to Duke Stefan—that he heard a bell ringing for victory, and that the spider was in the end "only a spider". The former could conceivably be a good guess on his part thanks to knowing the story of the bell of Prince Paul like every other Varanian—like the Delphic oracle to Darius, he could claim a bell would ring for victory no matter who actually won. But how did he know thatthe spider was hidden near a real spider web, thus making it "only a spider"?
Diego Manolos, Con Man and blackmailer, turned out to be The Man Behind the Man when it came to manipulating and controlling Ruffino, and his hand could be said to be indirectly behind everything that happened in Haunted Mirror. Manolos, meanwhile, is rather reminiscent of the Spanish word for hand, "manos".
Roger Callow of Dead Man's Riddle—he is described as young and somewhat inexperienced as a lawyer, which matches his last name perfectly.
August August of Fiery Eye.
Bert Clock, professional screamer who liked to collect clocks, and eventually had them all modified so as to scream in place of an alarm—and his nickname was also Screaming Clock.
Professor Shay of Phantom Laketurns out to be a bit shady.
The Meddling Kids Are Useless: Unlike The Hardy Boys and Nancy Drew, completely averted. The boys are always instrumental to finding the treasure/heirloom/lost loot in question, with the cops instead showing up to be the muscle to arrest the villains and, often, providing information to the boys so they can solve the case. The few times where the cops and other adults are more proactive, the boys are still the ones to solve the riddle, provide the proof of the villain's scheme/identity, or explain the nature of the story's events.
Micro Monarchy: Varania of Silver Spider. Based on the descriptions (and illustrations) of the place, it seems mostly modeled off of Luxembourg or Liechtenstein, although the nature of the Minstrel Party, some of the naming conventions, and the presence of gypsies bring to mind The Trumpeter of Krakow and therefore Eastern Europe.
Mirror Scare: Unsurprisingly this happens with the "ghost" seen in the eponymous Haunted Mirror, complete with quite an example of a Nightmare Face (and accomplished by means of a well-hidden Bookcase Passage, of course).
Moustache de Plume: Author Mary V. Carey wrote under the name M.V. Carey, presumably because Random House thought the boy readers of the series wouldn't read books written by a woman.
Multilayer Façade: The villain of Crooked Cat, the Amazing Gabbo, uses one of these—wearing his clown costume over his regular old man appearance which is in turn a disguise hiding his true youthful self that he takes off to commit the Bank Robbery, but with the addition of a tattoo. It's also a form of Living a Double Life, since he's hiding his true self so as to appear as a harmless old man.
Mummies at the Dinner Table: Borderline example—while as far as we know Mr. Green of the Green Mansion never did this with his wife's corpse, he did stash her body in a secret room in his house, laid out in a coffin with her finest clothes and the Ghost Pearls.
Needle in a Stack of Needles: This nearly gets one of the Investigators killed. Jupe finds himself locked in the trunk of a car driven by a group of criminals, but managed to mark the floor of the garage the car will eventually return to with a large chalk "X", and even informs the other investigators of this via a walkie-talkie. Unfortunately, Jerk JockSkinny Norris was listening in and had his gang mark every garage they could get into just to be an ass.
Nephewism: Jupiter Jones lives with his aunt and uncle, his parents having died in an accident when he was very young. By contrast both Bob and Pete's parents are shown on numerous occasions, and sometimes even have significant roles in the stories.
Never One Crime: The villain in Invisible Dog commits quite the slew of crimes. Not only does he steal the Carpathian Hound and then extort money from its owner for its safe return, but he knocks out and nearly kills the church caretaker while hiding there, and after hiding the hound in the pool he poisons one tenant who takes midnight swims, plants a bomb under the hood of the manager's car because she was planning to have the pool drained and cleaned, and starts a fire in another tenant's apartment to give himself an alibi for the ransom exchange, being safely in a nearby hospital after feigning being overcome by the smoke. All of this, of course, was done so he could obtain the money to replace that which he'd taken from his nephew's trust fund and lost in stock market speculation.
Never Say "Die": Although the boys never really come too close to death, the danger they suffer is often very real and both they and their families worry about getting injured or killed. Of course the worst violence they usually suffer is getting knocked out and/or Bound and Gagged/Locked in a Freezer. But in Dead Man's Riddle they do almost go over a waterfall, and in the early book Green Ghost, when Jensen asks Mr. Won what to do if the boys don't turn over the Ghost Pearls, he coldly tells him to slit their throats. The threats Three-Dots makes with his Sword Cane in Fiery Eye (and his supposed You Have Failed Me killing of one of the Black Moustache gang) are taken quite seriously as well. They are also held at gunpoint several times (Stuttering Parrot, Vanishing Treasure, Screaming Clock, Laughing Shadow, Flaming Footprints, Shrinking House) and are left to die in the desert in Death Trap Mine.
Nobody Here but Us Statues: The burglar in Invisible Dog actually pulls off this one, disguising himself as a statue of St. Patrick in the neighborhood church using a pageant costume.
Nonindicative Name: Most of the books' titles (save some of the more modern entries in the series) had names which related directly to the plot, albeit sometimes in a subtle or more mysterious way, but at least one has generated some confusion due to its vagueness: The Mystery of the Vanishing Treasure. The book does involve a Bank Robbery and the theft of a priceless belt from a museum, but none of this is clear from the title alone.
Not Allowed to Grow Up: This is particularly noteworthy because it was attempting to age the boys up in one of the modern relaunches of the series that ultimately killed it in the United States.
Not in Front of the Parrot: Inverted in Stuttering Parrot. Parrots taught clues to the location of a hidden treasure have been so badly traumatized by their treatment at the hands of criminals that they refuse to talk. Fortunately, a mynah bird in the group (a last-minute replacement for a dead parrot) has memorized all the clues, and can be easily coaxed to repeat them.
Obfuscating Stupidity: Jupiter Jones, in many a case. He's the smartest person in the group, but because he is a former child actor and a little plump, he can act unintelligent in order to disarm people and get information from them that he wouldn't get otherwise. Every once in a while though a canny villain sees through the act.
Once an Episode: The boys will show their business card, with the slogan "We Investigate Anything", to someone, usually a prospective client. After Green Ghost, when their assistance proved invaluable to Chief Reynolds, they were given a card from him as well identifying them as junior deputies—something which was sometimes needed to convince clients that three boys could be real detectives, let alone useful ones. Also once an episode, there would be an introduction from Alfred Hitchcock (actually written by the author, and later replaced by Hector Sebastian) with a few teaser details about the case at hand and the rest being a nearly identical explanation of their background and how they got together. And there would always be a meeting with their mentor at the end, explaining details which couldn't be covered in the main narrative and otherwise wrapping the case up.
Orwellian Retcon: After 1980, the conceit of having Alfred Hitchcock introduce (through ghost writers) the books and interact with the boys was no longer feasible. As a replacement, the authors created a mystery writer named "Hector Sebastian." Some editions of the earlier books written with Hitchcock as a character replace him with Sebastian (or the fictional director Reginald Clarke).
Polish The Turd: The sole rationale for the series initially being named Alfred Hitchcock and the Three Investigators, and the frequent cameos of Hitchcock in the first thirty books or so, was Robert Arthur's insight that the books would sell better if they were somehow connected to somebody famous. He was right.
Portrait Painting Peephole: There's actually one of these in Terror Castle. Justified by both the nature of the plot (human agency being behind the haunting) and characterization (the castle's owner was a silent horror film star who built his home to be modeled off of his movies).
Put on a Bus: Despite seeming to reform after being used and abandoned by Marechal in Shrinking House, Skinny Norris almost gets the boys killed in Dead Man's Riddle and appears at his nastiest and the closest he comes to true criminal activity in Headless Horse (aiding and abetting Cody in concealing who started the brush fire and framing Pico for it). When the truth comes out he is sent away by his father to military school and never seen again in the series. Said by Word of God to have happened because Skinny's character was too flat, limited, and uninteresting to be useful to the series.
Real After All: Aside from the fact the monster of Monster Mountain turns out to be a genuine mountain man, several of the entries involving the supernatural written after M. V. Carey took over the series turned out to be real, or at least implied to be. In a chillingly effective moment at the end of Haunted Mirror, the villain sees something in the supposedly cursed glass that makes him flee right into the arms of the police; unable to explain it, the boys look very uneasily at the mirror and quickly leave. More obviously, when the villain of Magic Circle flees the scene only to crash in his car while the witch of the eponymous circle looks on with grim vindication, the boys have to wonder if she cursed him for what he had done to her; Jupiter scoffs at such notions of course, and a true Wiccan would not curse lest she run afoul of the Three-fold Rule, but... And in Invisible Dog, not only does Sonny Elmquist turn out to be a real astral wanderer, but it seems quite likely that the phantom priest seen in the church was a real ghost.
Reclusive Artist: Madeline Bainbridge of Magic Circle. Discovered young by a talent agent wowed by her beauty and instantly given a lucrative contract, her career reads like an Expy of Elizabeth Taylor—constantly in the public eye, famous and beloved across the country, star of numerous sweeping, big-budget, period epics, known for a coterie of fellow actors and other Hollywood notables, and involved in a public scandal due to romance on the set between herself and her male co-star. She never quite became a White-Dwarf Starlet, but her final film was widely derided as a ridiculous, overblown flop, and while it was Ramon Desparto's death which caused her to go into seclusion, it's possible her career might never have recovered from it. In any event, she cut herself off completely from the world, refusing all visitors, never watching television and rarely using the telephone, and allowing all of her business affairs to be handled by her manager/former chauffeur. This turned out to be a mistake.
The Red Baron: The Whisperer, aka Jonathan Rex from Terror Castle. Also, El Diablo of Moaning Cave.
Red Herring: Happens a lot—rarely ever is the boys' initial suspect the true criminal, nor is the thing they seek found in the first place they look. A particularly good one was in Death Trap Mine: the fact Mrs. Macomber had suddenly left her job, disappeared for several months, came into money out of nowhere when she had been destitute and forced to work at what had once been her own store, matched the description of a member of a holdup gang, and then vanished after a newspaper referencing the robbery was discovered all led Jupiter to believe she was a member of the gang and possibly the one who killed the man found in the mine. But instead she'd come into money when a relative died, and been kidnapped by Thurgood for recognizing he was an imposter, and the rest was all coincidence.
Another good example: Rory from Phantom Lake whose whole purpose in the plot, other than being a Violent Glaswegian, was to distract the reader from wondering who might really be Java Jim or his accomplice. On the one hand, his constant attempts to convince the boys to stop looking for the treasure, his conveniently timed comings and goings, and his various misdeeds all made a great candidate for the villain—too obvious, in fact. Which might cause some readers to get suckered into thinking he was guilty, while others would dismiss him but then be left not knowing who the real villain was. Anyway, he wasn't Java Jim—he just didn't want Mrs. Gunn to be rich because he was afraid she wouldn't accept his marriage proposal. A bit of Unfortunate Implications there—what she wants doesn't matter? She can only be rich if he earns it for her and takes care of her?—but otherwise a Crowning Moment of Heartwarming and Funny in one.
Regent for Life: A desire to make this happen kicks off the plot of Silver Spider.
Ranch: Headless Horse, complete with the evil developer being none other than Skinny Norris's Jerkass father (though he does at least confine himself to legal means, for the most part) and with the solution being finding a long-lost family heirloom. Proof that this trope really can work for any plot.
Scooby-Doo Hoax: The frequent explanation behind seemingly supernatural happenings. Textbook examples include Green Ghost, Skeleton Island, Flaming Footprints, Haunted Mirror (maybe), Dancing Devil, and Sinister Scarecrow.
Secret Passage: Terror Castle has two of them—one behind a mirror that leads to the projection room with its Ominous Pipe Organ and Blue Phantom, the other underground connecting the dungeons to Jonathan Rex's parakeet enclosure. A few other books have secret passages too, or at least secret rooms.
Sherlock Scan: Jupiter does this a few times, often when using a clue to deduce connections between suspects and the crime. Mocked by Bob on at least one occasion, in Magic Circle, when they happen to find the matchbook incriminating Harold Thomas.
Ship Tease: Between Jupiter and Allie, if their Slap-Slap-Kiss relationship (minus the kissing) is any indication. Perhaps a natural progression of putting two know-it-alls together.
There's a John Dickson Carr story where a killer hides a glass knife by dropping it into a jug of water. This was referenced in The Mystery of the Invisible Dog where a glass statue is hidden in a swimming pool. Jupiter Jones figures it out by remembering the Carr story. See Hidden in Plain Sight.
In The Mystery of the Laughing Shadow, no one who hears the eponymous shadow's laugh can identify it or describe it, and in fact it sounds different to everyone who hears it. This is a reference to Poe's "Murders in the Rue Morgue" where no one could identify the language of the murderer because he wasn't speaking one at all, being an orangutan. Jupiter again references the parallel, and it is apt since the shadow's laugh is actually made by a kookaburra, thus identifying the resident Evil Aussie as the Big Bad.
In The Mystery of the Fiery Eye, Jupe deduces that the eponymous jewel is hidden in one of a series of plaster busts based on the fact their owner had been a Sherlock Holmes fan and one story, The Adventure of the Six Napoleons, had a valuable object hidden in similar fashion. The whole line of investigation, though, turns out to be a bust, since it's a Red Herring.
Bob mentions, when they are discussing the possible hiding places of Angus Gunn's Pirate Booty in Phantom Lake, that it could be hidden right in front of their face, something they look at all the time but never even notice or think about, like the purloined letter of Poe. He's absolutely right too, since the hiding place for the treasure is the lake itself, or its island anyway.
Robert Arthur did a Shout-Out to himself in Silver Spider: when the time came for Jupe, Bob, and Rudy to escape from the dungeon with a The Guards Must Be Crazy plot (specifically, the sick prisoner ploy), Jupe offered an idea he'd read in a story from an Alfred Hitchcock collection, that of making ropes from torn sheets, with nooses at each end, and slipping them over the guards' ankles and necks so that, the more they struggled to get free, the tighter they became entangled with each other. Said plot is indeed from an Alfred Hitchcock book, specifically his "Solve-Them-Yourself Mysteries"—and it was written by Robert Arthur.
Silver Spider also contains a religious Shout-Out: in the backstory of Varania's Prince Paul, it's revealed that the reason he made the spider a protected creature (and his personal symbol) was because while fleeing from revolutionaries, he went to hide in an attic, and a spider there formed a web over the door so that when the revolutionaries searched the house, they assumed no one had been in the attic in a long time since they’d have to have torn through the cobweb to get in, and so his life was spared. This is nearly identical to a story about King Saul and David, as well as one told of Muhammad.
Shout-Out to Shakespeare: Aside from the usage of Hamlet's soliloquy in Stuttering Parrot, there's the identity of Puck/Robin Goodfellow in Magic Circle and the quote from Macbeth in Dead Man's Riddle. Also, Jupiter's uncle's full name is Titus Andronicus Jones.
Sidekick: In many of the books, the boys have one of these in the form of their client, a local who happens to make a handy guide or Mr. Exposition, or a relative who either is seeking a lost heirloom/treasure or wishes help to Clear Their Name or that of a member of their family. A Running Gag early on is that this would be a boy of a new nationality for each book—Carlos from Stuttering Parrot, Hamid from Whispering Mummy, Chang from Green Ghost, Taro from Vanishing Treasure, Chris from Skeleton Island, August August from Fiery Eye, and Djaro from Silver Spider. This gag was dropped for a while, although sidekicks continued in Screaming Clock, Crooked Cat, Flaming Footprints, Nervous Lion, Singing Serpent (this time a girl!), and Shrinking House, then was briefly resurrected for Cluny of Phantom Lake (well, Scottish-American, but close enough) and Diego of Headless Horse. A number of these were found in and around Rocky Beach, justified by its proximity to both Los Angeles and Hollywood. Interestingly, none of these were reckless (at least no more so than the boys themselves) and many were quite helpful.
Amusingly, once the sidekick in question was their thirty-year-old publisher boss who was also their client (Magic Circle).
Spotting the Thread: Often used to catch the villain (or catch him in a lie), such as when the fake El Diablo in Moaning Cave was revealed to be Professor Walsh by the fact he held his gun in his left hand, or in Sinister Scarecrow when things like knowing about the crystal-hung candelabra on the museum staircase landing and somehow getting down the box of photographs from the closet shelf revealed that Mrs. Chumley could walk. But on at least one occasion it was a Red Herring—after having chased the villain into the barranca in Shrinking House and knowing he'd injured himself falling in, the boys looked for a limp to identify him later. But DeGroot's limp turned out to be from an old injury, and he wasn't even a villain.
Straw Vegetarian: Mr. Smathers of Monster Mountain. He's a bit more sympathetic than most examples; isn't weak, repressed and secretly desiring meat, or wishing all humans to be killed (in fact he's a pacifist); and the fact he turns out to be in the right and not an Evil Vegetarian is a sign the author actually agreed with his views but unfortunately overdid the depiction into caricature. But he's certainly quite strident in his views, pushes them on others, and has a rose-tinted view toward animals—though this may be slightly justified since he seems to be a genuine Friend to All Living Things.
Mr. Harris of Laughing Shadow is an even stronger example of this trope. As well as an Evil Vegetarian who, thanks to being a Con Man, is also a hypocrite since he's actually a regular meat-eater in disguise.
Sundial Waypoint: Used to find the eponymous Fiery Eye. Notable in that the waypoint is a mountain overlooking a canyon actually named (Sun)Dial Canyon.
Sword Cane: Three-Dots has one in Fiery Eye, which he uses to quite the chilling effect on the boys.
Teen Genius: Jupiter Jones. He has the general knowledge of an educated adult, often comes up with ingenious ploys, frequently builds technical gadgetry from scratch, and besides is a skilled actor. Stated to be something he developed through constant reading and research specifically to counteract his aversion to being mocked and laughed at for his weight.
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.
Theme Naming: The various entrances to the salvage yard and to Headquarters all follow one of two themes: Added Alliterative Appeal (Green Gate One, Red Gate Rover, Tunnel Two) or Rhymes on a Dime (Easy Three, Dour Foor). A numbering system is also included in the names, naturally.
Thirteen Is Unlucky: Invoked by Hitchcock in his introduction to Crooked Cat, wherein he notes the extreme difficulties, turns of bad luck, numerous accidents at the carnival, and other dangers that assault the boys which they might have avoided, or at least been better prepared for, if they'd been wary of taking on their thirteenth case. (Also note the case involves looking for a stuffed cat which, though striped, is mostly black.)
This Is Reality: Crops up every once in a while, such as in Fiery Eye when Jupiter is tied up by the villains and thinks, "In stories, when someone was tied up there was always a convenient way to get loose....But he had nothing."
Title Drop: Variation. Other than when the case name is mentioned by or to Hitchcock during the ending summation, and when the title refers to a place, it is otherwise never mentioned directly in the books. However there is usually at least one scene where the item or thing in question is referenced or described, with the unique trait or appended adjective mentioned (the parrot that stutters, the mummy that whispers, the clock that screams, the laughing shadow, the lion that's nervous, the headless horse, a scarecrow that's sinister, and so on). On one occasion however—the very first book!—Jupiter actually declares they must solve "the secret of Terror Castle".
Trademark: The three question marks on the Three Investigators' business card, standing for "questions unanswered, riddles unsolved, enigmas unexplained" and similar verbiage and designed by Jupiter to make people ask about their significance, and stand out in their minds. It works.
Two Lines, No Waiting: Occasionally a book will have multiple plotlines/cases, and quite often they intersect. In at least two books the lines literally intersected, with the midgets of Vanishing Treasure being responsible for the stolen belt, the "gnomes", and the bank robbery, and with Harold Thomas of Magic Circle responsible for the fire, the theft of the manuscript, and the theft of the films (albeit by working for two different criminals).
Villain Takes an Interest: In Vanishing Treasure, thanks to admiring Jupiter's intelligence, Rawley keeps offering to take him on as a criminal apprentice. Hugenay makes a similar offer in Screaming Clock.
Villain Team-Up: In Sinister Scarecrow, thanks to Blackmail. The more usual version (for money and/or equally criminal desires) appears in Vanishing Treasure and Magic Circle.
What Happened to the Mouse?: After the events of Stuttering Parrot, an afterword by Alfred Hitchcock stated that the young Mexican boy Carlos was taken under Worthington's wing, gotten a job washing and cleaning the cars at the Rent 'n' Ride Auto Rental Agency, and allowed to live with the Joneses, earning room and board by helping at the salvage yard. But the reader never sees or hears from him again in any of the later books. Additionally, the mynah bird Blackbeard is adopted by the boys as a sort of mascot for the Three Investigators, but after a few brief appearances he too vanished from the series—even from the books written by Arthur.
What's a Secret Four?: The boys' junkyard-based hideout has a large number of code-named entrances (Green Gate One, Red Gate Rover, etc.). Every mentioned path or doorway was used in at least one of the stories, except for "Secret Four".
Where the Hell is Rocky Beach?: Unlike most examples of this trope, the location of the fictional Rocky Beach is actually fairly easily guessed due to the specific details of landmarks and distance to other California cities. One fansite has pinpointed it to either Topanga Beach or Ocean Park, California, both within the right distance range of Hollywood, Santa Monica, and Los Angeles.
Working the Same Case: Two examples, both early in the series—in Whispering Mummy Pete, fed up with the seemingly supernatural case, decides to go off on his own to investigate a missing cat, only to find out it connects to the mummy. Then in Vanishing Treasure the bank robbery being performed by the "gnomes" they investigate turns out to be perpetrated by the same thieves who stole the Emperor's belt from the museum which had forbidden them from getting involved (because they were "just kids").
Written by the Winners: The Aesop of Headless Horse, as referenced by both Hitchcock and Jupiter. The three soldiers who, in their greed, decided to steal the Cortes Sword from Don Sebastian Alvaro made up a fake weapons-smuggling, fake breakout, and fake death for their prisoner, then deserted to pursue him; their report on this is what entered the history books and, without any other documentation or information to contradict, it was believed to be the truth not only by the Army but by Don Sebastian's son Jose. Helped by the fact that both Alvaro and the deserters died, so that no one remained who could tell what really happened.
You Meddling Kids: The villain of Crooked Cat comes this close to saying the trope name when caught! ("I'd have got away except for those stupid kids!") Few others do, though a number of them are certainly enraged or have breakdowns due to being caught by teenage detectives.
Your Princess Is in Another Castle: Fairly frequent, when either the lost item they're searching for literally does turn out to be somewhere else or they encounter a Red Herring. One particularly memorable example is in Stuttering Parrot when, after following every clue to the Merita Valley graveyard, they discover the long flat box which had once held the painting holding only a note saying, essentially, "You didn't read the clues well enough, better luck next time!" The last parrot clue, "I never give a sucker an even break, and that's a lead pipe cinch!" even lampshades this...until it turns out it was actually a stealth clue telling them the lead pipe found in the graveyard is the real hiding place for the painting. Another example in Fiery Eye, almost a comedy of errors, not only involves them looking in the wrong bust for the jewel, but finding a Fakin' MacGuffin version of it, followed by looking in the right bust after it got mistakenly switched with one that the villains stole but only finding another note telling them to "delve deeper". Still another example occurs with Dingo Towne and the fake stash of gems in Dead Man's Riddle.