Share one back office (database) over three drupal websites
Teeyo,

You can do this with the domains module that can be found here:
https://www.drupal.org/project/domain

The installation instructions can be found here:
https://www.drupal.org/node/477372

Don't forget to edit your settings.php file ;-)

I used the module myself a couple of times, so if you need any assistance, just let me know :-)