function updateBoxHeight() {
  var boxes = $$(".site").eachSlice(3);    
  boxes.each(function(group) {
    var h = 0;
    group.each(function(element) {
      h = Math.max(h, element.getHeight());
    })    
    group.each(function(element) {
     element.setStyle({height: h - 20 + "px"});
    })            
  })
}
Event.observe(window, "load", updateBoxHeight);