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 ROMs are probably known to you. It's possible you'll break something, and ROMs at this stage tend to have some bugs. The official CyanogenMod stuff tends to be at least to a certain level of stability, though. You'll also be assured of updates from time to time, and probably a build of CM12 when that's ready to go.
Just head to the CM download page and grab the 232MB ROM, but remember to find the Google apps package to install as well.