Apply Additional Discount after coupon

Put this code in your current theme function.php file. Set how many discount as per your require.

function add_extra_discount( $cart ) {

  $discount = $cart->subtotal * 0.3;

  $cart->add_fee( __( 'Extra Discount', 'twentyseventeen' ) , -$discount );

}
add_action( 'woocommerce_cart_calculate_fees', 'add_extra_discount' );