What are 3 codes hidden in 39 clues book 8 cover?

Answer 1

1 Because its there

2 GM

3 Henry

Also if you look in the book's inside cover, on the hat there are symbols. On the bottom of dedication page there is a key to decipher the message. the message reads " Tenzing knew too". Tenzing is the first climber to reach the summit of Mt Everest.