Divide Your Canvas Into 3 Columns Using Guides

If you want to divide your canvas into 3 equal columns using Guides, go into the View menu and choose “New Guide…” In the New Guide dialog box enter “33.33%” in the position field, and press OK. Then repeat the same process but this time enter “66.66%” in the position field.

An easier way to do this would be to use a plugin called GuideGuide, which allows you to easily create pixel accurate columns, rows, and midpoints using guides.