Taking the page width to 100%

Dear community,

I run the website https://www.espritcoubertin.com, I use WP 3.4 and I use the theme Arras. At the moment, I use the layout with a right sidebar only but I would like to have a left and a right sidebar, which I can easily do with Arras.

My problem is simple. When I activate the 3 column layout (left, middle and right), everything gets squeezed in what used to be the 2 column layout. To understand well, navigate to my home page. You will see that there is empty space on the left and on the right of the content. How can I use this space or extend my page so that when I activate the 3 column design, that space is used? I would like the 2 columns (right and left) to have the same width than what the right column has at the moment. The middle column with my slideshow will not be affected because the empty space will be filled with the new column.

I hope you understand what I mean! I know it has to do with CSS but I don’t know where and what to do. Thanks for helping me!