Making a div float "middle" between a left floating div and a right floating div

I have 3 divs inside a container. I would like 'contentpaneLeft' to float left, 'contentpaneRight' to float right, and 'contentpaneMiddle' to stay evenly between the two. Ideally the middle one would expand vertically if necessary. Is there anyway to do this in CSS without resorting to a table? I've attached my current CSS.

#container{width:100%; height:644px; padding:20px; margin:auto; background: white;}
.contentpaneLeft{width:250px; height:333px; float:left}
.contentpaneMiddle{width:480px; margin: 0 auto; height:644px;}
.contentpaneRight{width:250px; height:333px; float:right}

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John CarneyReliability Business Tools Analyst IIAsked:
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David S.Consultant & Challenge SubduerCommented:
The simple way to do that would be to give "#container" a minimum width and make sure that both the left and right columns come before the middle one in the markup.

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John CarneyReliability Business Tools Analyst IIAuthor Commented:
Perfect, Kravimir, thanks!

- John
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