Ok I found this code on the site
// Redefine woocommerce_output_related_products()
function woocommerce_output_related_products() {
woocommerce_related_products(3,3); // Display 3 products in rows of 3
}
and it worked on adding a 3rd product. But I'd like all 3 to be in one row. In other words, shrink the size of the thumbs.