CSS stylesheet tweak

Right now all pages are a 2 column layout. I would like to add the ability to have a 3 column layout on some of the pages, due to some new content to be added. OK, in detail. I have already changed all pages to a width of 910px - This stays Forum, Recommend, & Legal pages are 180px left column and 650 right column - This stays Index, Sales, Articles, Contact are currently 2 columns, I want them to become 3 column. So I can add some boxes on the right with test IPs, and other self advertising/promotional type material. So the site does a little more selling the product to the customer. I can add the boxes, once the CSS will support a third column. I understand how, and don't have the exact boxes yet. So really it is just altering the CSS style sheet so some pages can be 2 columns while others are 3. 3 column layout will be 180px / 440px / 180px (this should leave spacing between columns, and the edge about the same)