You can stack the blocks .span*inside the container .row-fluid, which can then be nested in another block span*to create the effect you are looking for. Try this for example:
HTML
<div class="container-fluid">
<div class="row-fluid">
<div class="span9">
<div class="big box">
box
</div>
</div>
<div class="span3">
<div class="row-fluid">
<div class="span3">
<div class="box">
box
</div>
</div>
<div class="span3">
<div class="box">
box
</div>
</div>
</div>
</div>
</div>
</div>
Notice how I put two side blocks on top of the other, contained in a container .row-fluidinside the other block .span*, to assemble them.
CSS .span* , :
CSS
.row-fluid [class*="span"] .row-fluid [class*="span"] {
margin-left: 0;
width: 100%;
}
: http://jsfiddle.net/k27S5/show/
HAML, :
HAML
.container-fluid
.row-fluid
.span9
content
.span3
.row-fluid
.span3
content
.span3
content