The section of your blog above the fold (what you visitor see without have to scroll down vertically) is the most important part of your blog. Businessmen like ground floor shop space, and that section above the fold is like ground floor shop space. Just as shoppers are lazy to walk up the stairs the second floor (the third floor would be worst), surfers are often lazy to scroll down your site to see the rest of the page and will quickly surf away if nothing catch their eye. So depending on what your objectives are, you should try to put what you want your visitors to see first above the fold. And the best way for you to do this is to use a 3 column template and not use a deep graphic Header that takes up most of the space above the fold. You may find it makes your blog attractive, but you will be unable to put many things above the fold.
Sometimes you are forced to compromise, like I compromised by putting a sitesearch box at the top of the main column above the post. But I feel it is useful because visitors can't miss that and hopefully they will use it to find topics in my blog and thus stay in this blog longer than otherwise. I think it is a worthwhile sacrifice. If you want a similar sitesearch box, you will need a Google AdSense account. You can apply for one via the "Generate revenue from your website. Google AdSense" text link at the top of the right sidebar of most of my blogs, including this one. (sorry, I got mixed up with my other blogs. I was forced to put it in the Header section because I set it so that visitors can search the web, this blog as well as 2 related blogs of mine. This made it too wide for it to fit into the main column)
If you want the sitesearch box for your blog, these posts may help you add implement it:
Add search box, AdSense ads, "stick post" at the top of main column (click BACK button to get back to this page)
Generate script for Google search box
Blending colors of your Google search box with the colors of your site
Add searchbox, graphic, AdSense ads unit or link unit, etc, to blog Header.
Plus here are some sources and demonstration blogs of 3 column New Blogger templates:
(click BACK button to get back to this page:
New Blogger template modified by Stavanger
Dots New Blogger template with Header
New Blogger templates modified by Ramani
New Blogger templates by Isnaini
New Blogger templates by Thur
Ramani's New Blogger Neo template
New Blogger template by Final Sense
If you want to do something
I think 3 column is useful as it helps you put more stuff above the fold (what you visitor see without have to scroll down vertically) that is helpful. I suggest you start using labels and then add a Label List to the first left column. Helps your visitors find relevant new/old posts quickly (I put Labels (Categories)) when I add a Label List as other bloggers using different blogging platform are more familiar with the term categories.
It is probably possible to get rid of one sidebar with some tinkering of the CSS and if you don't want to have an empty space in its place, you will have to change the main column and sidebar widths plus do some calculations. As I am a 3 column addict, I have no interest in trying and lots of other things to do. I hope you accept my suggestion above as I really think 3 column is very useful. Many surfers are lazy and if they don't see the thing they are looking for when they first land on your blog, they will quickly surf away without bothering to scroll down to see what else is there in the rest of the page below the fold.