Ulf Blanck 
The Three ???, Climbing Adventure (drei Fragezeichen) [EPUB ebook] 

It’s dangerous high up in the trees – especially when someone is manipulating the newly opened Monkey Mountain Climbing Park. Who is sawing into the planks and loosening the screws? The Three ??? are about to find out!
In Rocky Beach, California, live the three detectives: Justus Jonas, Peter Shaw and Bob Andrews. Together, they are The Three ??? and there is nothing they don’t take on! The three friends witness exciting detective stories, solve puzzles no matter how tricky, and can always rely on each other.