I have the same question. I have no idea where the right sidebar came from. It just appeared. It has some of the same items as the left side bar. I would like either to remove it or to be able to place the menu items I want but I do not know how where I can do this. The customizer only shows 2 side bars not 3.
Not sure that this is still of interest, but here’s a link to an article that covers it:
http://www.findurlaptop.com/tech/2014/03/04/remove-right-sidebar-in-twenty-fourteen-wordpress-theme/
The important thing if you want some pages to have the sidebar and others not to is to use different templates.