I thought it was supposed to be tercero, not tercer. How do you know when to use which one?
Before a noun, tercero loses its O (same goes for primero) (e.g. I am the first = Soy el primero; I am the first man = Soy el primer hombre)
In Europe 'tercer piso' is 4th floor... I anticipated what you would do with that and answered it as third floor... but that's not reality.
Thanks for the link, Daniel, but of course it doesn't answer the question: When is it better to use piso and when planta? Is the difference regional?