If you are comfortable using Drupal I’d suggest trying out Typo3. When I audited the two systems (as well as Joomla) I found that Typo3 is much easier to configure as it provides a lot more programming hooks. The learning curve is much steeper than Drupal though.

However I’d argue that Drupal in particular is also designed with true scalability and the programmer’s needs to the fore. And precisely because of this mix of power and popularity it can offer an unbeatable pool of modules – http://drupal.org/project/modules – either ready-to-use for the designer or fully customisable by the programmer.

And I’d say that most people would use “big” in terms of market share / popularity rather than power (eg car manufacturers, technology companies, actors on a TV show etc).

However you don’t give any hard facts to doubt “the big three’s” popularity or status (see the cms market share survey by water and stone based on parameters such as rate of adoption and brand strength which fully supports the received wisdom – “executive summary: three leading brands emerge” http://www.waterandstone.com/downloads/2008OpenSourceCMSMarketSurvey.pdf).

It’s true this argument becomes self-fulfilling – bloggers talk about the most popular products and so they become more popular – but that also becomes self-reinforcing. The biggest strength of an open-source cms is its community-provided modules (and support) so the bigger the community, the better the platform.

Drupal, Wordpress and Joomla might not be your big three but they are generally perceived as such and are getting bigger all the time.

If we presume though that there is a big 3 of CMS products then these three are not in it at all. Vignette, Typo3 and IBM Websphere are all vastly superior products on a technical basis. These are generally enterprise CMS packages, although I have deployed Typo3 on many smaller websites.

The three you have listed probably get the most press, therefore are most known, but that’s because they are bloggers buzzwords.