Checks to see if the hierarchical submenu returns anything. If true output the left column else give the content a full width class.

wp-check-sub-menu.php

  <?php
    $submenu = hierarchical_submenu($post);

    if (!empty($submenu)) {
      echo '
        <div class="sidebar col-3-12">
          '. $submenu .'
        </div>
        <section id="content" class="col-9-12" role="main">
      ';
    } else {
      echo '
        <section id="content" class="col-1-1" role="main">
      ';
    }
  ?>