function limit_types(the_form) {
  var elements = document.forms[the_form].elements['iam_id[]'];
  var elements_count = (typeof(elements.length)!='undefined')?elements.length:0;
  var count_checked = 0;
  for(var i=0;i<elements_count;i++)
    if(elements[i].checked)
      count_checked++;
  if(count_checked>3) {
    alert('Maximum 3 choices!');
    return false;
  }
}