When changing a value in a dropdown menu, the old

Question

I was helped out with this code (So thanks if it was you who helped me :P)

Currently, when you select an item in the dropdown box, it removes it from the others. But now, I also want it to add the item that was deleted form the others when you change the value...

Eg. You select 'One' in box 1 (now 'One' is removed from boxes 2 and 3.)

You now select 'Two' in box 1... I would like the previously selected 'One' to be re-added to the other boxes as now it's not in use.

Thanks (Not using jquery or prototype etc)

Javascript:

<script type="text/javascript">
function check(element)
{
  drops = document.getElementsByName('drop').length;
  opts = 4;
  for (i=1;i<drops+1;i++)
  {
    if (element.id != 'drop'+i && element.value != '0')
    {
      for (z=0;z<opts;z++)
      {
        if (document.getElementById('drop'+i).options[z].value == element.value)
        {
          document.getElementById('drop'+i).options[z] = null;
          break;
        }
      }
    }
  }
}
</script>

HTML:

<select id="drop1" name="drop" onchange="check(this);"> 
  <option value="0"></option>
  <option value="3">One</option>
  <option value="5">Two</option>
  <option value="1">Three</option>
</select>
<select id="drop2" name="drop" onchange="check(this);">
  <option value="0"></option>
  <option value="3">One</option>
  <option value="5">Two</option>
  <option value="1">Three</option>
</select>
<select id="drop3" name="drop" onchange="check(this);">
  <option value="0"></option>
  <option value="3">One</option>
  <option value="5">Two</option>
  <option value="1">Three</option>
</select>