You could do this with a Template Variable quite easily.
Create a new TV (for this example it’s named bodyclass)
Fill out the description and caption fields if needed.
Input Type: DropDown List Menu
Input Option Values: No Sidebars==no-sidebars||2 Sidebars==two-sidebars||3 Sidebars==three-sidebars
Default Value: No Sidebars (if left empty it will use the first in the above option values list)
Make sure you check the template(s) you want this TV available to.
Open your template(s) and add this: <body class="[*bodyclass*]">
Now for every document that uses this template variable, there will be a dropdown list of the classes which you can choose from.
Hope this helps