If I purchase this theme, where can I find tutorials on how to edit it? I'd like to make a 3rd column. Will it be too complicated?
I totally agree with Darryl Hein... Here is a veeery useful site to learn CSS.
At the Bottom of that article you will find some links to tutorials of "How To Create a Template from a PSD File" This one includes a description of how to create some variations for a template, maybe that can help you out :)