When I first came across Joomla a majority of the designs/templates out there consisted of a three column center focussed layout, and yes while there are still three coloumn templates being produced as we speak, for the most part modern Joomla template design has started to veer away from this.

Andy Rutledge has written a great article highlighting a flaw of the center focussed three column design. He suggests that the perceived benefits of the three column design such as maximising page space, centering the text and creating a symmentrical design aren't actually benefits when compared to a design thats lead by some basic design and content hierachy fundamentals.