timmmay: Unfortunately IMEI problems can serious. Disrespective managed to wipe his IMEI and had to buy a new phone. This is why you should always do two different IMEI backups before you mess with the firmware.
S3 should be fine with the latest firmware. In your place I'd probably upgrade to what it was before and see how it works. If the IMEI is blank I think the solution is to buy a new phone unfortunately.
It's likely you've bricked your phone if your IMEI has been reset.
S3 runs fine on the latest 4.3 firmware and is far faster and better than 4.1
Product Manager @ PB Tech
fahrenheit: If you have flashed 4.3, then you have a knox bootloader now. Warranty is void, so you will definitely be paying for any servicing.