lass="editable">
      .box {
        width: 500px;
        display: flex;
        flex-wrap: wrap;
      }
      .box > * {
        flex: 0 0 33.3333%;
      }
    </style>
  </head>

  <body>
    <section>
      <div class="box">
        <div>One</div>
        <div>Two</div>
        <div>Three</div>
        <div>Four</div>
        <div>Five</div>
        <div>Six</div>
        <div>Seven</div>
        <div>Eight</div>
        <div>Nine</div>
        <div>Ten</div>
      </div>
    </section>
    <textarea