indeterminate-box

  <form>
    <!-- カスタムフォームのチェックボックス -->
    <div class="custom-control custom-checkbox mb-3">
      <input type="checkbox" class="custom-control-input" id="checkbox08" checked>
      <label class="custom-control-label" for="checkbox08">選択済み</label>
    </div>
    <div class="custom-control custom-checkbox mb-3">
      <input type="checkbox" class="custom-control-input" id="checkbox09" unchecked>
      <label class="custom-control-label" for="checkbox09">未選択</label>
    </div>
    <div class="custom-control custom-checkbox mb-3">
      <input type="checkbox" class="custom-control-input indeterminate-box" id="checkbox10">
      <label class="custom-control-label" for="checkbox10">不確定</label>
    </div>
  </form>