The issue isn't sammobile. It is just that att never realeased a full firmware file beyond 4.1.1. All updates were pushed to i747 owners via Ota updates.
That's great. We're all here to help and learn from each other.
CWM is outdated in comparison to TWRP.
Here is the link for tar version of TWRP 2.8.70 for the ATT s3 (d2lte): https://dl.twrp.me/d2att/
I prefer Odin 3.07 for flashing: https://www.google.ca/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&...
Here's what I recommend:
1) download Odin 3.07 from here: https://www.google.ca/url?sa=t&rct=j...07763241,d.dmo
2) download the tar version of TWRP 220.127.116.11 from here: https://dl.twrp.me/d2att/
3) download the latest cm12.1 for the phone from here...
Does the phone have the latest version of TWRP installed? Is the card formatted as fat32?