Q:

How can I make all pages urls of my joomla 3 web site have the .html extension? 

How can I make URLs showing the .html extension in all my pages?  
For example, http://localhost/site/contacts.html 
I use joomla 3. 

A:

Here is how to show it:
For adding the document extension type in your URLs, you need to go in:
-> Global Configuration
-> Site
-> enable the "Add Suffix to URL" setting, at the SEO Settings section. 
Note that this may be overridden by rules in your .htaccess file, or 3rd party SEF extensions, so in case you have it enabled in Global Configuration but it doesn't show, you need to review those. 
Most people don't like the extension in their URLs, why you want to have it?