<ul class="">
    <li class="apple">リンゴ1</li>
    <li class="banana">バナナ1</li>
    <li class="melon">メロン1</li>
    <li class="apple">リンゴ2</li>
   <li class="banana">バナナ2</li>
   <li class="melon">メロン2</li>
   <li class="apple">リンゴ3</li>
  
</ul>

li:nth-child(3 of .apple) { 
    color: red;
  background:yellow;
}
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