Triple Spies (Boys Mystery Series, Book 1)

Roy J. Snell (1878-1959) authored at least 85 Young Adult novels under his own name and as by David O'Hara, James Craig, and Joseph Marino, most of them specifically directed to boys, though he wrote at least one series of mysteries for girls. His tales for younger children, beginning with Little White Fox and his Arctic Friends (1916), are animal fantasies.
This series consists of:
Triple Spies
Lost in the Air
Panther Eye
Crimson Flash
White Fire
Black Schooner
Hidden Trail
The Firebug
Red Lure
Forbidden Cargoes
Johnny Longbow
The Rope of Gold
Arrow of Fire
Riddle of the Storm
The Galloping Ghost
Whispers at Dawn
Mystery Wings
Red Dynamite
The Shadow Passes
Sign of the Green Arrow