I spent a full day searching for and trying out numerous tutorials and none of them worked! Until, I found Adding a Third Column to your Blogger Blog created by GardenBloggers.com. It's a simple, easy to follow tutorial with a 'find this piece of code' and 'change it to that piece of code' teaching style I love! You can view the end results of the 3 column blog tutorial on What Wrinkles? or Ontario's Cottage Country blogs anytime you like ;) but they've since been tweaked due to...
...and the even shorter route which enables 3 column, 2 column, 1 column layouts and more, at the click of a button!! I conveniently discovered this method the day after altering code via the above tutorial (go figure!)...
I'd suggest you save a copy of your blog first, directly to your computer, and then go into 'draft' mode.
The looks, layouts, options etc. are quite different (in a good way ;) so if you do end up messing around with some of these new choices you'll at least have the original you started with.
Good luck and please feel free to comment!