[Vlasenko Fedor]

<style>
      div.testblock {
        width:200px;
        margin: 10px;
        float: left;
      }
      div.testblock>textarea {
        width:200px;
        height: 80px;
        clear:booth;
      }
    </style>
    <div class="testblock">
      <textarea></textarea>
      <input type="button" value="Insert text" />
    </div>
    <div class="testblock">
      <textarea></textarea>
      <input type="button" value="Insert text" />
    </div>
    <div class="testblock">
      <textarea></textarea>
      <input type="button" value="Insert text" />
    </div>
    <script>
      window.onload = function () {
        var inps = document.querySelectorAll("div.testblock>input"),
          len = inps.length;
        for (; inps[--len].setAttribute("onclick", "ins(this)"), len;);
        ins = function (el) {
          var par = el.parentNode || el.parentElement,
            tar = par.children[0];
          tar.innerHTML += "Yes ";
        }
      }
    </script>

должно подтолкнуть на нахождение решения