I believe you are remembering a series called "The Three
Investigators." Some of the books in the series were printed under the
title "Alfred Hitchcock and the Three Investigators."
"The Three Investigators series, starring Jupiter Jones, Pete Crenshaw
and Bob Andrews. The original Scooby gang--but without the girls. More
fun than the Hardy Boys by far! Their clubhouse was hidden in a
junkyard and could only be entered by following cryptic messages, like
'Green Gate One' (which meant to enter through the secret door in the
part of a fence that was painted green).
These three guys had business cards that said 'The Three
Investigators: We investigate anything'."
Wet Noodle Posse: Nostalgic Reads
Here's a brief excerpt from "The Case of the Restless Gargoyle." The
text from which the excerpt was taken (in .pdf format) can be read by
clicking on the link below the excerpt (it may be a long load, because
it is an entire book):
"On the outside of the fence was a picture of a green ocean with a
two-masted ship riding the waves of a mighty storm. A fish had been
painted in the foreground of the picture, watching the foundering
vessel. An artist had made a knot in the wood the fish?s eye. When the
eye was pressed, it activated a catch that swung the two boards up.
This was Green Gate One, one of several secret entrances in and out of
the salvage yard."
Three Investigators Books: The Case of the Restless Gargoyle
More .pdf files containing the complete text of books in the series:
Three Investigators Books: The Secret of the Three Impostors
Three Investigators Books: The Mystery of the Hitchcock Inheritance
Three Investigators Books: The Adventure of the Bronze Claw
The .pdf files linked above came from this website, which is dedicated
to "The Three Investigators":
Three Investigators Books
If you are interested in purchasing the books, they are out of print,
but readily available on the used market. This search string will give
you a wide field of choices:
AddALL Rare, Used and Out of Print Book Search: The Three Investigators
My Google search strategy:
Google Web Search: "the three investigators" clubhouse OR "club house" fence
I hope this is exactly what you're looking for! If not, please request
clarification; I'll be glad to offer further assistance before you
rate my answer.