autocomplete not working in ci3?

Without more information, CI JSON support is probably not the problem. Assuming you're using AJAX POSTs, I can guess that either your CSRF protection is enabled and csrf_regenerate is set to TRUE or your sessions are now somehow interfering with your autocomplete code. If it's the former, you can either exclude your AJAX URIs in your csrf_exclude_uris setting or set csrf_regenerate to FALSE (if your application doesn't require tight CSRF protection).