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The Tercero Llave is Spanish for "Third Key". It is in the form of an old Mayan bowl from Francilee Jackson.
Scooby-Doo! Mystery Incorporated
Jackson had the bowl concealed in her Gluten Demon costume, and it fell out when she was captured.
It appeared with dirt being kicked into it by El Fuchi in the time-frozen scene of the Alianza Misterio found in the sitting room. (Nightmare in Red)
The gang packed it in a bag for their mission to find and destroy the Evil Entity.
It was required to open the door to the Earth Dimension. It had to be filled with dirt and used to balance a scale before the door would open.
- Like the other keys, the frozen scene associates it with a Greek element, in this case earth.