Having used all 3 at various points I have to say my "gut" instinct is that WP has a way larger community. I think just common sense tells us that WP is much more widely used simply due to the popularity of blogging. Joomla / Drupal kinda offer something different.

That said, does it matter? It's like people saying that iPhone beats Android because there are a gazillion more apps. Well so what when pretty much everyone just wants angry birds and facebook ;-)