Login using given credentials.

URL: http://natas3.natas.labs.overthewire.org
Username: natas3
Password: sJIJxxxxxxxxxxxxxxxxxxxxxxxxxxxx

The message on the page is:

There is nothing on this page

A look into the source code of the page shows a comment.

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<div id="content">
There is nothing on this page
<!-- No more information leaks!! Not even Google will find it this time... -->
</div>

Not even Google will find it this time... is a hint for the file robots.txt, which can tell search engines or crawlers what behavior the page owner wants on his homepage.
The content of this file is:

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User-agent: *
Disallow: /s3cr3t/

This means, that every User-agent is disallowed to crawl/list the folder s3cr3t. This directory contains the file users.txt with the password for the next level.

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natas4:Z9tkxxxxxxxxxxxxxxxxxxxxxxxxxxxx