I want to include a third sidebar on the left of the mainbody. It appears to do it until you save and close, at which point it reverts back to two sidebars and a mainbody.
to get a third sidebar you need to have modules published in
sidebar-a
sidebar-b
sidebar-c
Please note that you don't "set" a layout via the layout setting.
You must must think of it as "what happens if ..."
So if you click on "4" for example you tell the template the layout that should be active
if you have modules sidebar-a, sidebar-b and sidebar-c.
So if you click on "3" you tell the template the layout that should be active
if you have modules sidebar-a and sidebar-b (maincolumn counns as 1).
What you have for "3" does not have an impact what you have for "4".
So you can prepare the layout for many situations with just one slider element.