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 <div id="content_nest">
  <div id="content_left">a<br/>a<br/>a<br/>a<br/>a</div>
  <div id="content_right">c<br/>c<br/>c</div>
  <div id="content">b<br/><br/>b2</div>
  <div id="unten">&nbsp;</div>
 </div>

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body, html
{
	margin: 0;
	border: 0;
	padding: 0;
}

#content_nest
{
	background: lime; /* Testzwecke */
	padding: 5px /* für den mittleren */
}

#content_left, #content_right
{
	width:160px;
	padding: 2px;
	border: 1px solid #C5D8C0;
	border-right: 2px solid #B0B7AF;
	border-bottom: 2px solid #B0B7AF;
	position: absolute;
	margin: 5px; /* Durch absolute Positionierung wird die padding-Angabe des Containers ausgehebelt, darum nachhelfen */

}
#content_left
{
	top: 0;
	left: 0;
}

#content_right
{
	top: 0;
	right: 0;
}

#content
{
	background: #FFFFFF;
	background-image:url(http://www.isjetztegal.de/images/background.jpg);
	background-repeat:repeat-y;
	margin-left: 170px; /* Abstand nach links/rechts */
	margin-right: 170px;
	border: 1px solid red; /*Testzwecke*/
}

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    <div id="content_nest">
        <div id="content_left">a</div>
        <div id="content">b</div>
        <div id="content_right">c</div>
    </div>

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#content_nest {
	min-height:600px;
	height:auto !important;
	height:600px;	
	padding: 5px;
	border: 0px;
	margin: 1px;
	background: #FFFFFF;
}

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#content_left {
	float:left;
	width:160;
	padding: 2px;
	border: 1px solid #C5D8C0;
	background-color:white;color:black;
	border-right: 2px solid #B0B7AF;
	border-bottom: 2px solid #B0B7AF;
	overflow:hidden;
}

#content {
	background: #FFFFFF;
	background-image:url(http://www.isjetztegal.de/images/background.jpg);
	background-repeat:repeat-y;
	overflow:auto;
}

#content_right {
	float:right;
	width:160;
	border: 1px solid #C5D8C0;
	background-color:white;color:black;
	border-right: 2px solid #B0B7AF;
	border-bottom: 2px solid #B0B7AF;
	overflow:hidden;
}