If you give the contentpane a 100% width , this might lead to problems in some browsers because the total width can be  more then 100% (100% + the rightpane)

If you set the width of the table to 100%, the contenpane will expand automatically, since an empty pane gets a class and width 0  from default.css

/* used to collapse panes without any content in them */
.DNNEmptyPane
{
	width: 0px;
}

I think you can probably do this in CSS but you'll have to make a couple changes.

Put the divs in the correct order in your markup:

left
content
right

Then float: left - all of them.  Each will have to have a width but I think you can put a percentage or pixel width for left and content and width: auto for the right one.  It should collapse if there is nothing in it.

Good luck

BK