I did the following on 1.7.8.8:
Open file
/themes/classic/templates/catalog/listing/product-list.tpl
change:
{include file='catalog/_partials/products.tpl' listing=$listing productClass="col-xs-12 col-sm-6 col-xl-4"}
to
{include file='catalog/_partials/products.tpl' listing=$listing productClass="col-xs-6 col-xl-4"}
AND
In the file:
themes/classic/templates/catalog/_partials/productlist.tpl
change
{if !empty($productClass)}{$productClass}{else}col-xs-12 col-sm-6 col-xl-4{/if}{/capture}
to
{if !empty($productClass)}{$productClass}{else}col-xs-6 col-xl-4{/if}{/capture}
That worked for me.
Best regards,
Mikael