In components/com_k2/templates/default/generic.php:

	<div class="genericItemList">
		<?php foreach($this->items as $item_key => $item): ?>
		<!-- Start K2 Item Layout -->
		.........
		<!-- End K2 Item Layout -->
		
		<?php  if(($item_key+1)%3==0): ?>	
			<div class="clr"></div>
		<?php endif; ?>
		
		<?php endforeach; ?>
	</div>

	<!-- Pagination -->
	<div class="clr"></div>

In components/com_k2/css/k2.css:
div.genericItemView {width:33.3%;float:left;} /* this is the item container for this view */
div.genericItemIntroText {font-size:inherit;font-weight:normal;line-height:inherit;padding:0 10px 0 10px;}