Menu item hidden behind others

You are correct that this is definitely a CSS issue.

To get the 3rd level menu to show up fully, you’ll want to target it and move it X number of pixels from the left until it lines up correctly. If your sub-menu’s have a set width, then use that set width to move the 3rd level into the correct position.

For example if my sub-menu’s had a fixed width of 200px I’d do:

/* Target the 3rd level nav */
ul.menu ul.sub-menu ul.sub-menu{
  position:absolute; /* Position absolute so we can move the nav element */
  left: 200px; /* Move the nav element 200px from the left based on the width we set above */
  top:0; /* Align the tertiary nav list to the top of the parent menu list */
}

Check out the codepen here for a full working demo sans dropdown. http://codepen.io/iandbanks/pen/PwdKeB