Javascript - set focus on an element but after three letters

<SCRIPT LANGUAGE=JavaScript>
function focusOnField(sField)
{

// Returns the element whose ID is specified.
var tr=document.getElementById(sField).createTextRange();
tr.moveStart('character',3);
tr.select();
// document.getElementById("elementName").focus();
}
// Ende -->
</script>
<input name="elementNamelogin" value="ABC" size="8" maxlength="8" onkeyup="focusOnField(this.form,this);">