错在哪里了？！
如题，请帮忙指教
<html>
<head>
<script language = "javascript">
<!--
function go(o,n){
if(o.value.length >= 3 && n != null)
document.myForm.elements[n].focus();
doValidate();
}
function doValidate(){
var isValid = true;
for(var i = 0;i < document.myForm.length;i++){
var items = document.myForm.elements[i];
if(items.value.length == 0){
isValid = false;
break;
}
}
if(isValid){
document.myForm.submit.disabled = false;
}
}
//-->
</script>
</head>
<body>
<form name = "myForm">
<input type = "text" name = "text1" size = "3" onkeyup = "go(this,1)">
<input type = "text" name = "text2" size = "3" onkeyup = "go(this,2)">
<input type = "text" name = "text3" size = "3" onkeyup = "go(this)">
<input type = "submit" value = "submit" disabled>
</form>
</body>
</html>
