$(document).ready(function(){
    $('#main_menu li').hover(function(){
        $('.submenu',this).css("display","block");
    },function(){
        $('.submenu',this).css("display","none");
    });
    
    $('#main_menu li ul li').hover(function(){
        $('.third_menu',this).css('display','block');
    },function(){
        $('.third_menu',this).css('display','none');
    });
    
});