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right side of the right page on a red piece of paper below the helmet and above the rabbit.
The third Juan is written on a pencil on page 10 at the bottom left corner. Sitting on the chalk ledge.
It's on the u on the page A is for
The blue thread in the I Spy School Days book can be found on the blue baseball cap on the bottom right corner of the "Art Class" spread.
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The third spy has it
On top of the red train
It's in the nest with the shell in it
The third in the series is Spy Goddess: The Spy Who Totally Had a Crush on Me.
The third spy is in the green house, at the top of the tallest building.
there is no key for the 3rd spy
it is on the big yellow B above one of the balloons.