var cb = document.getElementsByTagName('input');
var checked = 0;
function countCB() {
	for(i = 0; i < cb.length; i++) {
    		if(cb[i].type == 'checkbox' && cb[i].checked) {
        		checked = checked +1;
    		}
  	}
  	
	if (checked == 0) {
  		alert ('U moet minstens 1 leverancier selecteren');
		checked = 0;
  		return false;
  	}
  	if (checked > 3) 
  	{
  		alert ('U kunt maximaal 3 leveranciers vergelijken');
		checked = 0;
  		return false;
  	}
	
	checked = 0;
  	return true;
}