Controlling sub-menu within sub-menu

Yes, you can easily control just using css.
Complete guide about wp nav menu on wordpress codex. And my fast solution for this:

functions.php

add_theme_support('menus'); // Menus Support
register_nav_menus(array('main_menu' =>'Main Menu')); // Main Menu of Website

header.php

<nav id="menu">
    <?php wp_nav_menu(array('theme_location'=>'main_menu')); ?>
    <div class="clear"></div>
</nav>

style.css

/* Clearfix */
.clear {clear:both}

/* First Level */
nav#menu ul li {float:left; position:relative; text-transform:uppercase; list-style:none}
nav#menu ul li a {display:block; background:#152635; color:#fff; padding:5px 15px; font-size:14px;}

/* Second Level */
nav#menu ul li ul {display:none;}
nav#menu ul li ul li a {width:160px;}
nav#menu ul li:hover ul {display:block; position:absolute; top:26px;}

/* Third Level */
nav#menu ul li ul li ul li {list-style:inside square}
nav#menu ul li ul li ul li a {font-size:11px; color:#ddd}

Obviously you need edit the CSS according to your interest. Enjoy!

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