Root Odin ?

[kennnxmondina]

I tried downgrading my at&t galaxy s3 , and when I did , there was no sounds , imei unknown . So lets get to what happened . I tried installing Custom Roms , and it would always fail in twrp . When I tried rebooting , it says " Firmware Something encountered , please connect to kies " I did , but it couldnt detect my phone . I tried connecting to odin , it connects , but when I tried flashing , it says "COM operation [write] failed" . UGH , HELP!!! [emoji24]

 

[mikedt]

Can lose IMEI when something is seriously borked with the low-level firmware, and if it's too damaged Kies may not even be recognizing it as a Samsung device.

I don't have an S3 myself so can't really help here. I've moved your question to S3 All Things Root, see if we can get some S3 experts to help you.

 

[funkylogik]

Try reflashing the stock (current) firmware with Odin. Hopefully that will restore your EFS file (IMEI)
EDIT just read you've tried that. Keep trying and make sure Kies isn't running while you use Odin