在一阵头痛之后，我设法使它发挥了作用。这是我的代码：

add_action( \'woocommerce_cart_calculate_fees\', \'iom_add_custom_discount\', 10, 1 );
function iom_add_custom_discount( $wc_cart ){
    $discount = 0;
    $product_ids = array();
    $product_ids_disc = array();
    $item_prices = array();
    $in_cart = false;
 
    foreach ( $wc_cart->get_cart() as $cart_item_key => $cart_item ) {
        $cart_product = $cart_item[\'data\'];
 
        // here we check if the products in the cart are in category \'detski-bodita\'.
        // since these are variation products, we check if their parents have the category.
        // if they do we add them the $in_cart boolean to true.
        if ( has_term( \'detski-bodita\', \'product_cat\', $cart_product->get_parent_id() ) ) {
            // var_dump(count($in_cart));
            $in_cart = true;
            $product_ids_disc[] = $cart_product->get_id();
            $item_prices[$cart_product->get_id()] = $cart_product->get_price(); 
        } else {
            $product_ids[] = $cart_product->get_id();
            $item_prices[$cart_product->get_id()] = $cart_product->get_price(); 
        }
    }
    // here we check if we have products with $in_cart boolean to true 
    // and if they are in category \'detski-bodita\'
    if ( ( $in_cart ) && ( has_term( \'detski-bodita\', \'product_cat\', $cart_product->get_parent_id() ) ) ) {
        $count_ids = count($product_ids_disc); // We count the items we have
        asort( $item_prices ); // Sort the prices from lowest to highest
 
        $count = 0; // we add another counter for the products that will be discounted.
        // here we check if the minimum amount of products is met for the discount.
        if( $count_ids >= 3 ) { 
            foreach( $item_prices as $id => $price ) {
                if( $count >= 1 ) {
                    break;
                }
 
                //$product = wc_get_product( $id );
                //$price = $product->get_price();
                $discount -= ($price * 100) / 100; // this is the discount that we apply - 100%
                $count++; // increase the counter in order to stop after the max amount of discounted products
           }
       }
 
    }
 
    if( $discount != 0 ){
        $wc_cart->add_fee( \'Отстъпка\', $discount, true  );
        # Note: Last argument in add_fee() method is related to applying the tax or not to the discount (true or false)
    }
}

我还有一些问题需要解决。仅当购物车中有3件相同类别的商品时，才应使用折扣。例如，类别为t恤和连帽衫。如果我的购物车里有3件t恤，就应该打折。如果我有两件t恤和一件连帽衫，则不应享受折扣。如果我有3件t恤和1件连帽衫，则应享受折扣。但是，我的功能似乎只有在购物车中有来自同一类别的3种不同产品时才起作用。如果我有1件数量为2的t恤和1件数量为1的t恤，那么它不起作用，应该在什么时候应用折扣。此外，如果我在购物车里放了3件t恤和1件连帽衫，然后取下1件t恤，那么在不应该的时候，折扣仍然适用。。任何帮助都将不胜感激。谢谢！：）