<script>
  function fun1()
  {
    var t1 = document.getElementById('t1').value;
    var t2 = document.getElementById('t2');
    //t2.value = t1.toLowerCase();
    //t2.value = t1.toUpperCase();
    t2.value = t1.slice(0, 3);

  }
</script>
<form>
  <input type="text" id="t1" value=""><br>
  <input type="text" id="t2" value=""><br>
</form>
  <button type="button" name="button" onclick="fun1()">OK</button>