For your scenario I would put all the css and image files in a common location except for the stylelayout.css which is where you would set the column widths. Then each version of the skin would have style.config, layout.master, theme.skin, and style layout.css (which would be different in each variation)
Keep in mind that the center gets assigned different css classes dynamicly and if the left or right has a different width then the center may also need to be changed depending on whether the layout uses absolute positioning for the left and right (in which case the margins on center have to adjust accordingly) or floated columns. Basically all the included skins use one or the other approach to column layout.
center-nomargins is assigned to divCenter if there is no content in left or right
center-rightandleftmargins is assigned to divCenter if there is content in both the right and left columns
center-rightmargin is assigned to divCenter if there is content only in center and right columns
center-leftmargin is assigned to divCenter if there is only content in the center and left columns
Hope it helps,
Joe