Embora não tenha percebido bem o que queres, penso que seja isto:
<script type="text/javascript">
function toUnicode(elmnt,content)
{
if (content.length==elmnt.maxLength)
{
next=elmnt.tabIndex
if (next<document.forms[0].elements.length)
{
document.forms[0].elements[next].focus()
}
}
}
</script>
E depois metes as inputs/textareas que quiseres, tipo:
<form name="form" form methode="post" value="active.asp">
<input size="4" tabindex="1" name="first"
maxlength="4" onkeyup="toUnicode(this,this.value)">
<input size="3" tabindex="2" name="Second"
maxlength="3" onkeyup="toUnicode(this,this.value)">
<input size="2" tabindex="3" name="Third"
maxlength="2" onkeyup="toUnicode(this,this.value)">
<br>
<br>
<input type="submit" name="submit" value="submit"
</form>