It took a while to get the bootloader situation figured out, but there are finally some ROMs showing up for the LG G3 on AT&T. There’s even a build of CyanogenMod 11, which was just posted today. It’s a nightly, but it’s still official support.
If this intrigues you, the consequences of flashing around with ROMs is probably known to you. It’s possible you’ll break something, and ROMs at this stage tend to have some bugs.
AT&T’s LG G3 (D850) Gets Its First Official CyanogenMod Nightly was written by the awesome team at Android Police.