1. You need to root and disable Knox (I saw you rooted but did you disable knox?)
2. You need a custom recovery. A quick way todo this is download this http://techerrata.com/file/twrp2/d2a....0.0-d2att.tar
(its the newest TWRP) and flash it using Odin.
3. then you should be able to flash which ever rom you want, BUT be careful because I think some can mess up the i747m. (PS cm11 isnt that great, still very buggy and battery life is not as good as stock.)
Oh one more thing. the CM11 file you have is old since it says d2att, cm renamed the device to d2lte which supports all north american galaxy s3's I believe.