How to make a two column posts list into three column in WordPress?

As you can see in the following theme : Mag-Grid

The posts in the latest articles are in three columns. But after I customized it and increased the featured image size(both in height and width), it came down to two columns and now shows like this : http://ift.tt/19fSiKW

I tried increasing the width of the container but it doesn't work.