Step 1: Login to your Blogger blog.
Step 2: Click > Layout > Edit HTML
Step 3: Don't forget to backup your current template just in case! Click "Download Full Template" and save it to your computer.
Step 4: Highlight your entire code and delete it, so your template is blank. Then open this document. Rosie Posie 3 Column
Step 5: Copy all the code and paste into your template. Preview and save!