RIFF JTAG for MF3...
According to the folks at the GSM forums, an update including the MF3 bootloader files has been added to the latest firmware for RIFF / JTAG repair. I have not tried this (I don't have a JTAG box) and I am not affliated with the people that provide this in any way. I also don't endorse anything, I am just relaying the information I have read. But for Developers or people with their own RIFF / JTAG boxes, this could be helpful. I am including a link to the post about the RIFF / JTAG MF3 update from the GSM forum, if it isn't allowed please feel free for a moderator to remove the link. Otherwise, you can go to the GSM forum and search for I337UCUAMF3 JTAG RIFF (or similar) and it should come up. Again, I have not tried this so if you decide to it is at your own risk. Good luck.
SGH-I337 (AT&T) 32GB
P-ROM (1st Gen.), De-Odexed, AMDL
Auto-LOKI OUDHS CWM
Make sure you read and understand the "PINNED" threads in the "General" and "Q & A" sections of the AT&T S4 forum BEFORE you ask for help. Use the forum's "search" function to find posts with additional information. You should not expect others to help you if you haven't tried to help yourself first. Also, read, understand, and follow the "XDA" rules.