The 2 Column is the default blog layout.
The sidebar can be on the right or the left.
It can also come on a wide version where the body post is wider.
The 3 column blog adds an extra sidebar.
This sidebar can be placed on either side of the body of your blog.
It can also be grouped next to the other sidebar on the left or right.
A 1 column blog is ideal if you use a lot of images. The widgets that are usually on the sidebar are placed below the body in the footer.