Whether you choose 2 or 3 column layout will be determined mostly by User Experience design, or in other words Visitor Experience design.
The 3 column layout might seem like a better option since we can put out more content, be that ads, recent news, or similar variations, in the sidebars. However the question remains who is to say that the more we have in the sidebar is better? When we consider a three column layout we do not necessary mean, the placement of the sidebars on the left and right side of the content, there are three column layouts where both side bars are situated on the right side of the content.
Sidebar. In a 3 column layout, each of the two sidebar columns is rarely as wide as the one sidebar column in a 2 column layout. Some of the broader widgets tend to be feel cluttered in such an environment. 2 column layouts have a little more defence in this regard.
Large content area. If you can pull of a large content area, be that larger pictures, better font choice, then having a two column layout can really highlight the content you want your readers to actually read. Which leads us to screen real estate.
Screen Real Estate. Many people now a days may not operate in an 800×600 pixel screen. However when constructing web layouts, designers should still take screen resolution into consideration. When there is a limited amount of screen real estate the importance and attention on content is a 2 column layout's strongest “selling point.”
Sidebars take up more space Whilst taking up less space can be an advantage, so too can taking up more space. When you have a lot of room to fill, and stretching your content horizontally can do damage to your layout, the extra sidebar width can be your saving grace.
The E-Commerce Advantage. Three column layouts are beneficial if you can assign a purpose to the three columns. A good amount of three column sites, today, fall into the e-commerce category. As it tends to give a shop a less cluttered feel as well as a way to market and highlight their products more effectively.
Centered content. With a 3 column design, it is possible to place one column on either side of the content, effectually centering the content column. This is especially useful if you want your readers to focus their eyes on the center content first before straying to the sides. Readers tend to focus on the center more in a three column layout as discussed in our F-Shaped Pattern article.
Therefore, the decision of a three column layout or two column layout will be based more on how you as the designer or the client wants to engage is visitors. If the objective is to engage them by content and articles, a two column may be more beneficial, if your planning to run a online store or have a lot of widgets to engage your visitor then you may lean more in favour of a three column design.