The only G3 with an unlocked bootloader is the D851 which is the T-Mobile variant. As the bootloader is not yet unlocked and no workaround has yet been discovered there is no possibility of flashing a ROM on any other variant.
I understand that you are new to XDA and are excited to start flashing ROMs but you should take a few minutes to familiarize yourself with the rules of the community and how the site operates. Your question easily could have been answered with a quick search. Good luck and welcome to the forums.