multiple widget use

Hi there,
I’ve written a really simple sidebar widget, here is the code:

function nt_adsense_3($args) {
	echo $args['before_widget'];
	echo '<div class="adsense_side">'.nt_option('nt_adsense_3').'</div>';
	echo $args['after_widget'];
}

wp_register_sidebar_widget(
    'nt_adsense_3',
    'Google AdSense Code 3',
    'nt_adsense_3',
    array(
        'description' => 'Google AdSense Anzeige für die Sidebar. Der Code kann in den Theme Einstellungen geändert werden.'
    )
);

I have three dynamic sidebars but I can only drag this widget to one of these sidebar areas. After dragging it to a sidebar it will disappear from the “available widgets” area.

What is to do to use this little widgets in multiple dynamic sidebars?

Best regards
Pascal