CSS:
#mega-menu-3 {
display: none;
}
JS:
$(window).load(function(){
$('#mega-menu-3').show();
});
If it doesn't work, try this (and ignore the CSS I suggested above):
$(document).ready(function(){
$('#mega-menu-3').hide();
});
$(window).load(function(){
$('#mega-menu-3').show();
});
Now it will only show when the page is fully loaded. Hope this helps. :)