I assumed by columns you're referring to, the left column being a widget area, the center column containing your post(s) and the right column being a widget area too ?
If that is the case, all you need to do is, go to one of your posts, then create that layout using the 3 column layout section. Once added, in the left & columns add either the Sidebar or Widgetizer section and in the center column add the Content/PostLoop section.
Once all the above has been completed, you should have your preferred layout, the only thing you will need to do now is reduce the size of the left and right column and increase the width of the center column, something like:
Left Column = 3/12
Center Column = 6/12
Right Column = 3/12
Now that you have your Post page configured, go to DMS Toolbar > Templates and save this template, call it Single Post. This will now effect all single post pages.