Drupal, though widely used throughout the Web is not as popular as the two mentionned above. Part of it probably comes from its steeper lurning curve.
Drupal is probably the most powerful CMS maintained by such a large community. The customization possibilities are almost endless with such a CMS. The complexity of Drupal structure is probably one of the biggest drawback of this CMS and often discourages non-web experts to use it to administer their website.