The center tall building on Balding Avenue. You can't get up to it from below, so you have to go up the building to the right of the Grease Monkey garage.
To get to the Third Spy
On Balding Avenue, use your grapple bowtie to go up the building to the right of the Grease Monkey garage. At the top, you can enter the "Rooftops". From there, you put on your Chameleon Suit and grapple left to the next building, which is guarded and has lasers to keep you from going up. Instead, go farther left to the red building, and climb that one. At the top, you can grapple right to get to the tower. Enter the tube to rescue the third spy from the Greenhouse.
(see related question)
The tallest building on Balding Avenue. It is where the third spy is held. (see related question)
Rescue the third spy from the Rooftops above the Grease Monkey garage. You will need the Grapple Bowtie from the second spy. (see related questions)
The third spy has it
The third spy is in the green house, at the top of the tallest building.
either in the building with all the Venus fly trap in the cage or in the building tied up in the in the attic
The second rescued spy (in the electrified building on Toupee Terrace) gives you the grappling bowtie, which you use to rescue the third and final missing spy.
there is no key for the 3rd spy
It is the Second Building on Main Street on Spy Island on poptropica.com.
The greenhouse is atop the tallest building on Balding Avenue, and the third spy is being held captive there. (see the related question)
There is no greenhouse on Spy Island.
Wear the chameleon suit. You will need to go farther left to the red building, then climb to the top antenna. From there you can grapple to the right to reach the taller building.
To rescue the second spy, avoid the electrified windows. To rescue the third spy, you will need to go up one building to the rooftops, then to the far left and back again. This requires the Grappling Bowtie from the second spy. (see the related question)