[G3 General] edmundopt:
Its January 2019 and I have purchased another G3, since I had to do this all over again, let me write how I did it :

from :

Installed Android Version : 6.0

security patch 2016-08-01

kernel 3.4.0

V30n-EUR-XX

to :

CleanRom 3.2

V30g-GLOBAL-COM

Steps:

1--> install ROM V30B (MM - MarshMallow), this root method only works with MM

1.1 --> install LG Mobile Driver into windows PC

1.2 --> download LG FLASH TOOL into windows PC

1.3 --> connect phone to computer and install ROM V30B

2--> ROOT the device

2.1 --> copy kingroot(.apk), AutoRec(.apk), twrp 3.2(.img), cleanRom 3.2(.zip) to the phone

2.2 --> install KingRoot

2.3 --> install AutoRec (TWRP)

3-->Update TWRP and install CleanROM

3.1 --> install TWRP 3.2.3

3.2 --> install CleanROM

STEP 1 : Install Stock V30B MarshMallow ROM

download ROM from : http://storagecow.eu/index.php?dir=Xda%2FLG+G3%2FStock%2FD855%2F

file : D85530B_00_1217.kdz

download LG FLASH TOOL from : http://www.mediafire.com/file/fwrcd3pdj0svjtb/LG+Flash+Tool+2014.zip

download and install into windows PC : LG Mobile Driver from : https://www.lg.com/us/lgeai/Drivers/pages/mobile-drivers.html

file : LGMobileDriver_WHQL_Ver_4.2.0

Install the LG driver, extract contents of LG Flash Tool 2014.zip into a folder, copy D85530B_00_1217.kdz into that folder, power off the phone.

To enable download mode in the phone, press VOLUME UP and connect the phone to the windows PC via usb cable, when the phone displays downlod mode you can leave it.

Start LGFlashTool2014.exe, select the D85530B_00_1217.kdz, press CSE Flash

wait for the process to complete, phone will reboot, then it will boot into android and start optimizing apps

check phone software version , must be V30B

STEP 2 : ROOT the device

download 1__KingRoot4.5.6.apk and 2__AutoRecMMD855.apk from : https://mega.nz/#F!3woXDCSK!D70sB0o-PpyKBAAH2_ujYQ

download latest D855 TWRP .img from : https://eu.dl.twrp.me/d855/

file : twrp-3.2.3-0-d855.img

download CleanROM 3.2 from : https://xdaforums.com/lg-g3/development/d855-clean-rom-1-0-t3629917

file : D855_CleanROM_3.2.zip

Copy all files to phone (1__KingRoot4.5.6.apk;2__AutoRecMMD855.apk; twrp-3.2.3-0-d855.img;D855_CleanROM_3.2.zip)

Turn ON Wireless/ conect to an Internet source (required for KingRoot)

using the file manager install 1__KingRoot4.5.6.apk, you will be prompted to enable Unknow Sources (go to Setings --> Secutiry --> Unknown Sources (check it))

run KingRoot, atempt the root process and wait for it to complete

using the file manager install 2__AutoRecMMD855.apk

run AutoRec to install TWRP, press the large M button, phone will rebot into recovery mode

STEP 3 : Update TWRP and Install CleanRom 3.2

Its required to update TWRP before installing CleanRom 3.2

inside TWRP 2.8.7.0 (installed by AutoRec) choose :

Press Install -> Click on bottom left Images -> select twrp-3.2.3-0-d855.img

Choose Recovery, TWRP will be installed, go back to main menu

Choose Reboot -> Recovey (PHONE WILL REBOOT)

Press Wipe -> Advanced Wipe -> Dalvik ; Cache ; Data ; System

Go back to main menu

Install -> D855_cleanROM_3.2.zip

follow the instalation guide, JOB DONE!

for my phone I use :

--> DO NOT USE new gpu drivers

--> NEVER UPDATE magisk, keep 17.1

--> Universal Cust

--> NO AROMA

Thanks to : MESA for CleanROM 3.2

Thanks to tute123456 for this tutorial root-twrp-marshmallow-d855-30b

Also, this is a great video that I found with the complete rooting process : https://www.youtube.com/watch?v=8VOgM3jLas4