Chad Hovell

Select extra layouts from the post editor Screen Options menu. 
There are now 24 variations to choose from!

Three Column Screen Layout

Three, four and five column screen layouts for the post editor.

davidpaulsson

sakjur

A small but simple plugin that enables shortcode support to create columns directly in your posts/pages.
This plugin enables you to divide content in the TinyMCE editor into columns that are either one half, one third, two third, three fourth, one fourth, one fifth, two fifth, three fifth, four fifth, one sixth or five sixth wide.

byBrick Columns

A plugin that enables for column based shortcodes.

Three Column Screen Layout vs byBrick Columns