20. Menú vertical 3 niveles (no me funciona bien)

Fuente. http://www.cssscript.com/multilevel-accordion-menu-with-plain-html-css/

<nav class="nav" role="navigation">
  <ul class="nav__list">
    <li>
      <input id="group-1" type="checkbox" hidden />
      <label for="group-1"><span class="fa fa-angle-right"></span> First level</label>
      <ul class="group-list">
        <li><a href="#">1st level item</a></li>
        <li>
          <input id="sub-group-1" type="checkbox" hidden />
          <label for="sub-group-1"><span class="fa fa-angle-right"></span> Second level</label>
          <ul class="sub-group-list">
            <li><a href="#">2nd level nav item</a></li>
            <li><a href="#">2nd level nav item</a></li>
            <li><a href="#">2nd level nav item</a></li>
            <li>
              <input id="sub-sub-group-1" type="checkbox" hidden />
              <label for="sub-sub-group-1"><span class="fa fa-angle-right"></span> Third level</label>
              <ul class="sub-sub-group-list">
                <li><a href="#">3rd level nav item</a></li>
                <li><a href="#">3rd level nav item</a></li>
                <li><a href="#">3rd level nav item</a></li>
              </ul>
            </li>
          </ul>
        </li>
      </ul>
    </li>
    <li>
      <input id="group-2" type="checkbox" hidden />
      <label for="group-2"><span class="fa fa-angle-right"></span> First level</label>
      <ul class="group-list">
        <li>
          <li><a href="#">1st level item</a></li>
          <li><a href="#">1st level item</a></li>
          <input id="sub-group-2" type="checkbox" hidden />
          <label for="sub-group-2"><span class="fa fa-angle-right"></span> Second level</label>
          <ul class="sub-group-list">
            <li><a href="#">2nd level nav item</a></li>
            <li><a href="#">2nd level nav item</a></li>
            <li>
              <input id="sub-sub-group-2" type="checkbox" hidden />
              <label for="sub-sub-group-2"><span class="fa fa-angle-right"></span> Third level</label>
              <ul class="sub-sub-group-list">
                <li><a href="#">3rd level nav item</a></li>
              </ul>
            </li>
          </ul>
        </li>
      </ul>
    </li>
    <li>
      <input id="group-3" type="checkbox" hidden />
      <label for="group-3"><span class="fa fa-angle-right"></span> First level</label>
      <ul class="group-list">
        <li>
          <li><a href="#">1st level item</a></li>
          <li><a href="#">1st level item</a></li>
          <input id="sub-group-3" type="checkbox" hidden />
          <label for="sub-group-3"><span class="fa fa-angle-right"></span> Second level</label>
          <ul class="sub-group-list">
            <li><a href="#">2nd level nav item</a></li>
            <li><a href="#">2nd level nav item</a></li>
            <li><a href="#">2nd level nav item</a></li>
            <li>
              <input id="sub-sub-group-3" type="checkbox" hidden />
              <label for="sub-sub-group-3"><span class="fa fa-angle-right"></span> Third level</label>
              <ul class="sub-sub-group-list">
                <li><a href="#">3rd level nav item</a></li>
                <li><a href="#">3rd level nav item</a></li>
                <li><a href="#">3rd level nav item</a></li>
              </ul>
            </li>
          </ul>
        </li>
      </ul>
    </li>
    <li>
      <input id="group-4" type="checkbox" hidden />
      <label for="group-4"><span class="fa fa-angle-right"></span> First level</label>
      <ul class="group-list">
        <li>
          <li><a href="#">1st level item</a></li>
          <input id="sub-group-4" type="checkbox" hidden />
          <label for="sub-group-4"><span class="fa fa-angle-right"></span> Second level</label>
          <ul class="sub-group-list">
            <li><a href="#">2nd level nav item</a></li>
            <li><a href="#">2nd level nav item</a></li>
          </ul>
        </li>
      </ul>
    </li>
  </ul>
</nav>

CSS

 .nav a, .nav label {
  display: block;
  padding: .85rem;
  color: #000;
  background-color: #fff;
  box-shadow: inset 0 -1px #1d1d1d;
  -webkit-transition: all .25s ease-in;
  transition: all .25s ease-in;
}

.nav a:focus, .nav a:hover, .nav label:focus, .nav label:hover {
  color: rgba(255, 255, 255, 0.5);
  background: #030303;
}

.nav label { cursor: pointer; }
/**
 * Styling first level lists items
 */

.group-list a, .group-list label {
  padding-left: 2rem;
  background: #252525;
  box-shadow: inset 0 -1px #373737;
}

.group-list a:focus, .group-list a:hover, .group-list label:focus, .group-list label:hover { background: #131313; }

/**
 * Styling second level list items
 */

.sub-group-list a, .sub-group-list label {
  padding-left: 4rem;
  background: #353535;
  box-shadow: inset 0 -1px #474747;
}

.sub-group-list a:focus, .sub-group-list a:hover, .sub-group-list label:focus, .sub-group-list label:hover { background: #232323; }

/**
 * Styling third level list items
 */

.sub-sub-group-list a, .sub-sub-group-list label {
  padding-left: 6rem;
  background: #454545;
  box-shadow: inset 0 -1px #575757;
}

.sub-sub-group-list a:focus, .sub-sub-group-list a:hover, .sub-sub-group-list label:focus, .sub-sub-group-list label:hover { background: #333333; }

/**
 * Hide nested lists
 */

.group-list, .sub-group-list, .sub-sub-group-list {
  height: 100%;
  max-height: 0;
  overflow: hidden;
  -webkit-transition: max-height .5s ease-in-out;
  transition: max-height .5s ease-in-out;
}

.nav__list input[type=checkbox]:checked + label + ul { /* reset the height when checkbox is checked */
max-height: 1000px; }
label > span {
  float: right;
  -webkit-transition: -webkit-transform .65s ease;
  transition: transform .65s ease;
}

.nav__list input[type=checkbox]:checked + label > span {
  -webkit-transform: rotate(90deg);
  -ms-transform: rotate(90deg);
  transform: rotate(90deg);
}