I created a simple Twenty Eleven child theme that adds 3 extra sidebar layouts to Twenty Eleven, all three columns. left right sidebars, 2 left and 2 right.
The sidebars will display on the single post page.
The new sidebar will follow Twenty Elevens fluid layout.
The comments were also modified to make better use of the narrow content area.
You can download it here http://zeaks.org/three-column-twenty-eleven/