<ul>
  <li><a href="#">Primary</a></li>
  <li><a href="#">Primary</a></li>
  <li><a href="#">Dropdown</a>
    <ul>
      <li><a href="#">Secondary</a></li>
      <li><a href="#">Secondary</a></li>
      <li><a href="#">Slider</a>
        <ul>
          <li><a href="#">Tertiary</a></li>
          <li><a href="#">Tertiary</a></li>
          <li><a href="#">Tertiary</a></li>
          <li><a href="#">Tertiary</a></li>
        </ul>
      </li>
      <li><a href="#">Secondary</a></li>
    </ul>
  </li>
  <li><a href="#">Primary</a></li>
  <li><a href="#">Primary</a></li>
</ul>

ul {
  margin: 0;
  padding: 0;
  list-style: none;
  overflow: hidden;
}
ul:hover {
  overflow: visible;
}
li {
  position: relative;
  float: left;
  background-color: #17B0FF;
}
li:hover {
  background-color: #189BDF;
}
a {
  display: block;
  padding: 10px 20px;
  color: #fff;
  font-weight: bold;
  text-decoration: none;
}

/* second level */
ul ul {
  max-height: 0;
  transition: max-height 1s;
}
ul li:hover ul {
  max-height: 999px;
}
ul ul li {
  float: none;
  background-color: #189BDF;
}
ul ul li:hover {
  background-color: #1289C7;
}

/* third level */
ul ul ul {
  position: absolute;
  width: 0;
  top: 0;
  left: 100%;
  transition: width 0.5s;
}
ul ul li:hover ul {
  width: 100%;
}
ul ul ul li {
  background-color: #DF7F18;
}
ul ul ul li:hover {
  background-color: #C77115;
}