If you want to build a grid is best that every 3 span4 you generate a new row-fluid below the other. For example:
<div class="row-fluid">
<div class="span12"> //These two row are our flexible container
/ GENERATE WITH YOUR CODE THIS STUCTURE UNDER EVERY 3 POST /
<div class="row-fluid">
<div class="span4"></div>
<div class="span4"></div>
<div class="span4"></div>
</div>
I use the php for doing it, but I think that is a small error of logical structuration of your html in the cycle!