Many moons ago, LG had promised the release of an official bootloader unlock tool for its G3, but the company didn't make good on its word. It seems that the plans were never forgotten though, because LG's Developer site got a new "Unlock Bootloader" page that does just that, except it's not for the G3 (yet).
While bootloader unlocking is quite easy on Nexus devices, almost all other Android phones and tablets suffer from tightly locked bootloaders that require a bit of tinkering to get access to. Both Sony and Motorola actively try to help developers achieve unlock (and there was a time when HTC did so too), but owners of other brands are usually left on their own - free to roam the forums and try to find a method that works, or to use Sunshine if it's compatible. And LG devices haven't been the most developer-friendly until now, heck most of them still aren't, but that new page is a good sign for the future.
LG's page details the risks of unlocking your bootloader, especially in losing your warranty and maybe raising vulnerability prospects, then proceeds to let you know that the only currently supported phone is LG's G4 for the EU open market, but that they "will add more devices with unlockable bootloaders unlocking permission progressively." Whether that means new devices going forward or that you could see old phones and tablets join the ranks (like the G3) remains a mystery.
Detailed step-by-step instructions follow on the page, with links to download the required files from Android Developer, how to request the unlock key, and instructions to get through the process. Once that's done, you should have an unlocked bootloader and all the freedom to install custom ROMs and mod your device to your liking.
- LG Developer Unlock Bootloader