Adds a third column on the Edit Post screen.
No, you don't need to modify WordPress at all.
The layout of boxes is specific to the post type, so you can have one layout for Posts, one for Pages, and one for each custom post type.
Open up the "Screen Options" panel in the top-right. Under the "Screen Layout" section, set the "Number of Columns" option to 3.
The boxes should all jump back to the second column.
If it doesn't work - if your boxes disappear - try turning the plugin on again, then back off.