The Three Investigators were a trio of boys who became involved in investigations through the patronage of Alfred Hitchcock. I loved the early adventures when I was a kid... I read the first dozen or so in Armada paperbacks although these had covers by Peter Archer rather than Harry Kane, who was the artist for the original hardcover editions published by Random House which were reused for the Collins hardcover editions. Roger Hall only produced internal illustrations.
If you remember the Three Investigators, there are plenty of websites about them across the internet. These covers came from here via Mark West's website. There is some fascinating material about the background to the series here, including material about the authors and correspondence between the original author (Robert Arthur) and the original publisher.