var MAXLEN = 3;

function autoTab(id, suffix){
 var curr = document.getElementById(id);
 
 if(curr.name=='mn1' && curr.value.length >= 3){
  curr.blur();
  document.getElementById('stph_mn2'+suffix).focus();
 } else if(curr.name=='mn2' && curr.value.length >= 3){
  curr.blur();
  document.getElementById('stph_mn3'+suffix).focus();
 }
}