Easily adjust your templates Column Layout via the Templates parameters which can be accessed by navigating to Extensions >> Template Manager >> JoomlageXXXX_YourTemplate.
From your 'Side Column Layout' dropdown simply set the layout of your side columns with options for both your side columns to appear on either the left or the right or on either side of your content/component area.
For a single column layout simply disable all modules in the SideColumn A and SideColumn B module positions. See all possible Column Layouts in image below.

Custom Page and Column Widths

Your Side Column Width can also be customised via the Templates parameters. Place the desired width of your Side Columns in the fileds provided. Your content area will automatically contract/expand to fill the remaining available area.