function prepare(oTxt1, evt){
  if (evt.ctrlKey) {
    var oFrm = oTxt1.form;
    oTxt1.maxLength = oTxt1.maxLength + oFrm.campo2.maxLength + oFrm.campo3.maxLength; //expand the maxlength of field 1
  }
}

function autoPopulate(oTxt1, oTxt2, oTxt3, maxlen1, evt){ //need to pass the original maxlength of oTxt1 since it is expanded when CTRL key is pressed
  var maxlen2 = oTxt2.maxLength;
  var maxlen3 = oTxt3.maxLength;
  var s = oTxt1.value;
  var len = s.length;
  if (len<maxlen1){
    return;
  }
  else {
    var midmax = maxlen1 + maxlen2;
    var totalmax = midmax + maxlen3;
    oTxt1.value = s.substr(0, maxlen1);
    if (len>=midmax){
      oTxt2.value = s.substr(maxlen1, maxlen2);
      if (len>=totalmax){
        oTxt3.value = s.substr(midmax, maxlen3);
        oTxt3.focus();
      }
      else if (len>midmax){
        oTxt3.value = s.substr(midmax); 
        oTxt3.focus();
      } 
      else {
        oTxt2.focus();
      }
    }
    else if (len>maxlen1){
      oTxt2.value = s.substr(maxlen1); 
      oTxt2.focus();
    }
  }
  oTxt1.maxLength = maxlen1; //change back to orig maxlength
}