:nth-last-of-type()

dt {
  font-weight: bold;
}

dd {
  margin: 3px;
}

dd:nth-last-of-type(3n) {
  border: 2px solid orange;
}

<dl>
  <dt>Vegetables:</dt>
  <dd>1. Tomatoes</dd>
  <dd>2. Cucumbers</dd>
  <dd>3. Mushrooms</dd>
  <dt>Fruits:</dt>
  <dd>4. Apples</dd>
  <dd>5. Mangos</dd>
  <dd>6. Pears</dd>
  <dd>7. Oranges</dd>
</dl>

:nth-last-of-type(<An+B> | even | odd) {
  /* ... */
}

<div>
  <span>This is a span.</span>
  <span>This is another span.</span>
  <em>This is emphasized.</em>
  <span>Wow, this span gets limed!!!</span>
  <del>This is struck through.</del>
  <span>Here is one last span.</span>
</div>

span:nth-last-of-type(2) {
  background-color: lime;
}