These are the steps i used to root my LG G3 running official marshmallow. I rooted my LG G3 d851 and d850 by this method and i am sure that this will work on most variant (Not sure about Sprint or korean edition but you can give it a try).
But first Disclaimer
I take no responsibility to/for bricked devices. This is simple guide to help rooting and installing TWRP. If everyone ended up with a soft or hard brick then i am not responsible for anything. Off-course you can ask for help but can't force or annoy me. Anyways good luck with that. Have a Happy rooting :cool:
Steps (Please do in sequence to avoid problem)
1: Download the official king root apk from here.
2: Also download correct twrp for your device
3: Now root your device
a: Install kingroot apk. b: Connect to internet and run the kingroot application
c: Let the application do it work by clicking on root now.
(It can display cannot root your device sometime but forget it. After process is complete the device would be rooted temporary)
4: As device is rooted temporary so don't restart it.
5: Make sure adb is setup properly on your pc if not then follow the guide
6: Now copy your download twrp recovery (of step 2) to your device and name it twrp (With extension it would be twrp.img).
7: Connect your device to pc (If it is not) in usb debugging mode.
Run "minimal adb and fastboot" (of step 5). Now type commands
adb shell su
dd if=/sdcard/twrp.img of=/dev/block/platform/msm_sdcc.1/by-name/recovery exit exit
adb reboot recovery
If you had anything else then ask in comments.
Hit thanks if this helped you