Thanks to awesome developer we know by the name of Chainfire, rooting the Marshmallow builds is simple and easy once again. Using Chainfire’s auto-patcher SuperSU build, you can root your LG G3 easily.
First make sure that you have the Marshmallow update on LG G3 (here) installed and running.
All you need for this is a custom recovery like TWRP recovery, or Cyanogen recovery. Download the TWRP recovery for your LG G3 model no. D855 from here.
Once you have installed TWRP recovery, download the SuperSU file from above, and install it using TWRP recovery.
For that, transfer the SuperSU file to your device, and then reboot into recovery mode to access TWRP recovery. Once in TWRP, tap on Install button, then select the zip file and then do the swipe action at the bottom to confirm. When done, tap on ‘Reboot System’ button to restart the device.
That’s it. Your LG G3 will be rooted.