function checkSize()
{
  var left = document.getElementById('left').offsetHeight;
  var right = document.getElementById('right').offsetHeight;
  var middle = document.getElementById('middle').offsetHeight;
  
  if ((left > middle) || (right > middle))
  {
    var max = (left > right)? left : right;
    document.getElementById('middle').style.height= (max + 120) + 'px';
  }

   window.setTimeout(checkSize, 1500);
}
window.setTimeout(checkSize, 1500);