var leftcolheight = document.getElementById('leftcol').offsetHeight;
var centercolheight = document.getElementById('centercol').offsetHeight;
var rightcolheight = document.getElementById('rightcol').offsetHeight;

if (leftcolheight < centercolheight) {
  document.getElementById('leftcol').style.height = centercolheight + 'px';
}else {
  if (leftcolheight < rightcolheight) {
    document.getElementById('leftcol').style.height = rightcolheight + 'px';
  }
}