Generated by SassMeister.com.

SassMeister-input-HTML.html

<div class="menu">
  <ul class="active">
    A
  </ul>
  <ul class="submenu">
    B
    <ul>
      <div>
      <li>
        B1
      </li>
      <li>
        B2
      </li>
      <li>
        B3
      </li>
      </div>
    </ul>
  </ul>
  <ul class="submenu">
    C
    <ul>
      <div>
      <li>
        C1
      </li>
      <li>
        C2
      </li>
      <li>
        C3
      </li>
      <li>
        C3
      </li>
      <li>
        C3
      </li>
      <li>
        C3
      </li>
      </div>
    </ul>
  </ul>
</div>

SassMeister-input.scss

// ----
// libsass (v3.2.5)
// ----

* {
  box-model: border;
}

.menu > ul {
  display: block;
  float: left;
  width: 33%;
  text-align: center;
  padding-left: 0px;
  position: relative;
  transition:all cubic-bezier(0.250, 0.460, 0.450, 0.940) 0.5s;
}
.menu > ul.active {
  background-color: #CCC;
}

.menu > ul:hover {
  background-color: #ddd;
}

.submenu > ul {
  display: none;
  float:left;
  list-style-type: none;
  padding-left: 0px;
  position: absolute;
  width: 100%;
  transition:all cubic-bezier(0.250, 0.460, 0.450, 0.940) 0.5s;
}
.submenu ul div {
  border: 1px solid green;
}
.submenu:hover > ul {
  display: block;
}

SassMeister-output.css

* {
  box-model: border;
}

.menu > ul {
  display: block;
  float: left;
  width: 33%;
  text-align: center;
  padding-left: 0px;
  position: relative;
  transition: all cubic-bezier(0.25, 0.46, 0.45, 0.94) 0.5s;
}

.menu > ul.active {
  background-color: #CCC;
}

.menu > ul:hover {
  background-color: #ddd;
}

.submenu > ul {
  display: none;
  float: left;
  list-style-type: none;
  padding-left: 0px;
  position: absolute;
  width: 100%;
  transition: all cubic-bezier(0.25, 0.46, 0.45, 0.94) 0.5s;
}

.submenu ul div {
  border: 1px solid green;
}

.submenu:hover > ul {
  display: block;
}

SassMeister-rendered.html

<div class="menu">
  <ul class="active">
    A
  </ul>
  <ul class="submenu">
    B
    <ul>
      <div>
      <li>
        B1
      </li>
      <li>
        B2
      </li>
      <li>
        B3
      </li>
      </div>
    </ul>
  </ul>
  <ul class="submenu">
    C
    <ul>
      <div>
      <li>
        C1
      </li>
      <li>
        C2
      </li>
      <li>
        C3
      </li>
      <li>
        C3
      </li>
      <li>
        C3
      </li>
      <li>
        C3
      </li>
      </div>
    </ul>
  </ul>
</div>