var groups = {};
$(".product-list .product-list_item").each(function(i) {
    var c = $(this).attr("class");
    if(!groups[c]) groups[c] = [];
    groups[c].push(this);
});
for(var i in groups) {
    $(groups[i]).wrapAll('<div class="product-list_row" />').each(function(i){
        if (((i+1) % 3) == 0)
            $(this).after('</div><div class="product-list_row">');
        });
    }