Adding a 3rd column to your blog is appealing to most bloggers. There are a few ways to go about it. The first option is the easiest, but comes with it's drawbacks, and that is to copy and paste a 3rd party's HTML pre-designed template. These layouts come with or without additional design, but almost always include a link on your blog that will take your readers back the blog designer's blog.
The other option is to change your own HTML. I used this website to create my third column.