For AT&T LG G3, Korean LG G3, and D855, you can also use TowelRoot, which is much easier than this root method!
You may be able to try the root method anyways even if it’s not listed but please do at your risk! (It should be safe to do but I will not be responsible for loss of data!)
Step 2. Hit the Build number about 6 or 7 times until it says “You are now a developer!”.
Step 3. Next hit the Back button twice and you will see a new menu called “Developer options”. Select the new menu.
Step 4. Make sure “USB debugging” is checked ON.
Step 5. Connect a micro-USB cable from your LG G3 to your computer.
Step 6. If using Mac or Linux skip this step. For Windows, open Device Manager and make sure you see “LGE Mobile Sooner Single ADB Interface”.
If you don’t see it, download and install LG G3 drivers:
Download LG G3 driver
Step 7. Download fastboot.zip and purpledrakeasset.zip.
Unzip both files and you should get a folder called “fastboot” and “purpledrakeasset”.
Copy the files from “purpledrakeasset” into “fastboot” folder.
Step 8. Open a command prompt for Windows or Terminal Emulator for Mac or Linux. (If you don’t know how to do this, please Google it!)
chmod 755 *