You’ll see the option to show each of the sidebars with a check box. You can also select their size.
Let us run through a couple different scenarios:
1) If I have the right sidebar showing and the size is set to 3 my main container will automatically calculate to ‘col-md-9’ to fill out the remaining columns of the grid to make them equal 12.
2) If I uncheck the “show” options for both sidebars the main container will have a column size of 12 (col-md-12).
3) If I check the boxes to show both sidebars (Right and Left) the main container size will be the remainder of columns left from 12 minus the sizes of the left and right sidebar added together.
So changing this logic sort of takes away from the auto-magic of it being an option to show the sidebars and select their size. But if you really wanted to mess with it look inside theme-options.php and the main functions file for a function called, “devdmbootstrap3_main_content_width_columns” this is the function that tells the templates what size to make the main container.