All 3 are excellent open-source content management systems built using PHP & MySQL. While none are a bad choice, I break it down like this.
WordPress is by far the most user friendly. So if you want to do some of the tasks by yourself, look no further. Theme and plugin development is at an all time high. If you need your website to do something, chances are there is a plugin for that purpose. WordPress is also the most search engine friendly right out of the box.
Joomla is also excellent, and user friendly, but it has not grown as much as WordPress. 3 years ago or more I would have leaned towards recommending Joomla, but now WordPress has so many more plugins and themes available plus WordPress has much better SEO capabilities. Another negative of Joomla is limited support for ACL (Access Control List). ACL grants certain privileges for certain users. This eliminates Joomla for enterprise level customers.
Drupal is a top-notch CMS, and the oldest of the 3 having started back in 2001. Drupal is the most powerful and the most technical of the 3, and has the steepest learning curve. If your company employs development staff to make most of the changes and to add functionalty, then this is a good way to go. Be aware there are very few free modules (Drupal uses modules rather than plugins). Also there are very few themes.
