By the end of this video your content area and its two sidebars are equal thirds instead of the default narrow-wide-narrow arrangement, and you know what happens when only one sidebar has blocks in it.
The default, and where it comes from
The main container holds three regions: Left Sidebar, Content and Right Sidebar. The theme does not use a fixed grid — a preprocess function counts which of the three actually contain blocks and assigns W3.CSS column classes accordingly. With both sidebars filled, each gets w3-quarter and the content gets w3-half, which is the 25 / 50 / 25 proportion people recognise. With no sidebars at all the content takes w3-col and runs full width.
The setting
Expand Main Container Columns' Width - W3CSS Theme in theme settings and tick Change the columns in the main container to (33% 33% 33%). That single checkbox is the whole feature.
What the checkbox actually changes
- Both sidebars in use: all three regions become
w3-third. - One sidebar in use: that sidebar becomes
w3-thirdand the content becomesw3-twothird. - No sidebar in use: nothing changes — the content stays full width.
So the choice is really about how much room a single sidebar is allowed to take: a quarter, or a third. Wide menus, faceted filters and advert blocks usually want the third; a short list of links does not.
If your screen looks different
Older documentation for this setting tells you to type 33-33-33 into a text box. In the current release it is a checkbox, and the underlying setting key is unchanged, so an existing configuration keeps working. If your site shows a text field, you are running an older build of the theme rather than doing it wrong.
The same proportional logic governs the top, bottom and footer containers, which divide their width by the number of regions you have filled — three filled footer regions become thirds automatically, with no setting to change. More detail on the W3CSS Theme project page, which runs on Drupal 9 through 12.
If this is one piece of a bigger build, Drupal architecture services is where that conversation starts — or get in touch.
