1) This rooting guide works for the korea version of LG G3 (F400K/L/S), Might works for D855.
2) Not works for the US version of LG G3 with a locked bootloader.
What You need:
To root your LG G3:
1. Install LG Universal Mobile Driver
2. Enable USB Debuging on Your LG G3
Setting -> About phone -> Software infomation -> tap 7 times on Build number
Go back Setting -> Developer options -> check to enable USB debugging mode
3. Connnect computer to LG G3 with USB Cable
4. Extract the just downloaded roottool.zip to “c:\lgtool”
5. Open Command Prompt, click Start, click Run, type cmd, and then click
6. From command promopt screen, navigate to “c:\lgtool”, Run Run.bat and then press any key to go to Recovery
7. On Android system recovery (3e) screen, Chose apply update from ADB in recovery
8. Then press any key from command window
9. press reboot
Now Your G3 is rooted.