All the included skins already handle this problem gracefully. If you put content on the page in the right pane the center pane will make room for it. If there is no content on the right the center will take up the available space. So in most cases you only need one skin.
In the case where you do really need a different skin set the main skin as the default site skin and set the home page to use a specific skin.
Hope it helps,
Joe