JavaScript is your friend! More specifically jQuery is your friend.
You can us wp enqueue script and do something like this:
Add this to your themes functions.php
<?php
function my_scripts_method() {
wp_enqueue_script('jquery');
}
add_action('wp_enqueue_scripts', 'my_scripts_method'); //
?>
Then either in your header or by writing a custom .js file then enquiring it use the following
jQuery(document).ready(function($) {
$('.hide_level3_button').click(function(){
$('.Level3').toggle();
return false;
});
});
Then just give your toggle button a class of hide_level3_button
and you’re good to go.
JS Fiddle: http://jsfiddle.net/BandonRandon/nzE7F/
UPDATE: You can also use fadeToggle()
to mak them fad out which may look better http://jsfiddle.net/BandonRandon/nzE7F/1/
EDIT
To add make it work on page load I would add the jQuery cookie plugin and change the javascript to this:
jQuery(document).ready(function($) {
if ($.cookie("hide_level3") == "true") { //check for cookie
$('.Level3').hide(); //hide the level
}
$('.hide_level3_button').click(function() {
$('.Level3').fadeToggle(); //toggle level
$.cookie('hide_level3', 'true'); //set the cookie
return false;
});
});