You have to find a blog layout that is made for three columns. There are lots of free ones online - for blog 2 I just did a google search for "blogger 3 column templates." When you find one you like they will provide you with the html you need to get it set up.It took me a while to figure out the coding, but there are lots of instructions online.
You can actually go to threecolumnblog.blogspot.com (I think that is the address). It has simple instructions you follow and voila! It was easy as pie and I am hardly a techy!
I was going to make the same suggestion as kappa prep. that site is so helpful!