var MAXLEN = 3;

function autoTab(name){
 var curr = document.getElementsByName(name)[0];
 
 if(curr.name=='mn1' && curr.value.length >= 3){
  curr.blur();
  document.getElementsByName('mn2')[0].focus();
 } else if(curr.name=='mn2' && curr.value.length >= 3){
  curr.blur();
  document.getElementsByName('mn3')[0].focus();
 }
}