Why is the blue div not adjacent to green div?
This is the html. Nothing but containers with borders.
<div id="wrapper960" style="min-height:350px; border:1px red solid">
<div class="content-sidebar-l" style="min-height:250px; border:1px yellow solid"> </div>
<div class="content" style="min-height:250px; border:1px green solid"></div>
<div class="content-sidebar-r"style="min-height:250px; border:1px blue solid"></div>
</div>
This is the CSS with centered div on the middle.
#wrapper960 {margin:0 auto; padding:0; width:960px;}
.content-sidebar-l {
float: left;
width:170px;
margin:0;
padding:0;}
.content {
margin:0 auto;
padding:1em 0 0 0;
width:610px;
background-color:#fff;}
.content-sidebar-r {
float: right;
width:160px;
margin:0;
padding:0;}

(source: imagesup.net)
Here it is in a JSFiddle
Re-order your HTML to:
jsFiddle example
You need to float your sidebar to the right first, otherwise by placing it last it can’t float up above an element before it that is also floated.