function h_dopdown_cat() {
    echo preg_replace_callback( '/(<option[^>]*>)((?:&nbsp;)+)/u', function ( $matches ) {
        return $matches[1] . str_repeat( '–', strlen( $matches[2] ) / 18 ) . ' ';
    }, wp_dropdown_categories( [
        'echo'         => 0,
        'depth'        => 3,
        'hierarchical' => true
    ] ) );
}

class My_Walker_CategoryDropdown extends Walker_CategoryDropdown {
    public function start_el( &$output, $category, $depth = 0, $args = array(), $id = 0 ) {
        $pad = str_repeat( '-', $depth );

        /** This filter is documented in wp-includes/category-template.php */
        $cat_name = apply_filters( 'list_cats', $category->name, $category );

        if ( isset( $args['value_field'] ) && isset( $category->{$args['value_field']} ) ) {
            $value_field = $args['value_field'];
        } else {
            $value_field = 'term_id';
        }

        $output .= "t<option class="level-$depth" value="" . esc_attr( $category->{$value_field} ) . """;

        // Type-juggling causes false matches, so we force everything to a string.
        if ( (string) $category->{$value_field} === (string) $args['selected'] ) {
            $output .= ' selected="selected"';
        }
        $output .= '>';
        $output .= $pad . $cat_name;
        if ( $args['show_count'] ) {
            $output .= '&nbsp;&nbsp;(' . number_format_i18n( $category->count ) . ')';
        }
        $output .= "</option>n";
    }

}

wp_dropdown_categories( [
    'depth'        => 3,
    'hierarchical' => true,
    'walker'       => new My_Walker_CategoryDropdown(),
] );