Looking on the internet deeply has found these results:

on the thrid floor is the most popular phrase on the web.

at the third floor 125,300 results on the web	on the thrid floor 116,300,000 results on the web More popular!
Some examples and use cases from the internet:	Some examples and use cases from the internet:
It doesn't look like this phrase is very popular! The search could not find any examples on the internet.	At the thrid charge I was forced towards enemy lines.

at the third floor

125,300 results on the web

on the thrid floor

116,300,000 results on the web

More popular!

It doesn't look like this phrase is very popular! The search could not find any examples on the internet.