The only way to make a content or div fill the space is to tell the side columns to be a static width--
(#sidebar {position: absolute; width: 100px; top:0; left:0;})
and then tell the content column to have a margin of the same amount as the width of the sidebar--
(#content {margin: 0 0 0 100px;})
By doing this, the #content column will automatically stretch to fill the page, but will not put any content where it's margins are defined.
By combining this with some clever PHP code (I borrowed of course), I have managed to have it count the number of "Joomla!" content modules in the sidebar columns so that if there are none, it applies a CSS snippet that resizes the page accordingly.
BTW... two divs existing on the same line with the "width: auto" doesn't work; one always pushes the other underneath it. The only way I know of (besides the way described above) is to give the divs widths that will allow them to all fit on the same line--
#column1 {width: 20%; float: left;}
#column2 {width: 60%; float: left;}
#column3 {width: 20%; float: right;}
OR you can give them static widths (e.g. 100px), but if the page width goes under the cumulative width of your columns, one or more columns will be pushed down.
Hope this is helpful to someone somewhere.
Thanks for you guys' help!