Split div into three equal parts: snippet 2

gistfile1.html

<div id="main_div">
  <div id="left_pane">
    <div id="first_left_pane">
      <!-- your left content goes here -->
    </div>
    <div id="second_left_pane">
      <!-- your middle content goes here -->
    </div> 
  </div>
  <div id="right_pane">
    <!-- your right content goes here -->
  </div>
</div>