If you need to add class to a single product in woocommerce category page, simply go to plugins/woocommerce/templates and copy the file content-product.php to yourtheme/woocommerce
On line 30, you will see the opening <li tag. This is the start of every product in the category page. Simply enter your tag in post_class and you will have your class appears on the category page.
<li <?php post_class("col-md-4 col-xs-6"); ?>>