Hi guys,
I see many users that are confusing when rooting their device.
@odjinan asked me for a clearer tutorial, so I'll provide it.
PLEASE NOTE that my post won't absolutely subtract credits to @tute123456, the starter of this thread.

BE CAREFUL if you're owning different/branded G3 variants, good result is not guaranteed and you may get in bootloop, hard brick in worst cases.
To prevent compatibility issues with latest stock-based roms (like Fulmics) is highly recommended to flash latest firmware (at least based on Marshmallow).
Ps despite my warnings, I'll purpose generic links for mad users, but please do not complain any bootloop, any kind of brick or explosion of your device if you're not owning D855.
You MUST OWN the D855 variant of LG G3, since there are no mirror link for Marshmallow AutoRec outside D855.

1. For noobs: to be sure you're owning the D855 variant of your device, go in "Settings > Device info > Hardware info" and look for "Model name": it should be "LG-D855".
   Then verify if you're already using a Marshmallow firmware: go back to "Device info", now on "Software info" and look for "SW version": it should be "30B" or higher value (30C, 30F etc.).
   
2. Update your device to latest firmware available.
   Are you on 30B firmware? DON'T UPDATE and ignore this step.
   Are you on newer firmware? Don't worry and downgrade: the procedure is almost safe, since I've tried it from 30O firmware without any issue.
   First of all download the proper firmware from here: you've to select the Global D85530B_00_1217.kdz file.
   When download is completed, shutdown phone and put it into Bootloader Mode: you have to simultaneously hold down the Volume- and plug device to computer via USB cable.
   Now download and extract LG Flash Tool 2014.zip, so you can start the homonym executable file: select the KDZ file previously downloaded, then choose the CSE Flash voice (it wipes data, as needed for downgrade).
   You'll see a prompt with country and language list: select the right settings and confirm, otherwise leave them unchanged.
   At this point you could be scared by interrogative characters and error messages: please ignore them and take a coffee
   If you're paranoid, look at phone display: the installation is going well
   
3. Download ad install latest KingRoot release: after opening it, attend for root permissions giving.
   
4. Now install the proper the D855 AutoRec (developed by @bender_007): download it from the MEGA link of main post.
   NOTE: don't panic if automatic reboot won't happen (as promised in app dialog), just do it manually: power off the phone, then press at the same time Power Button and Volume+, until you see LG logo.
   Ps because of an "aggressive" booting, you may get wrong and boot up normally device: don't worry and try again.
   
5. Now you're on recovery mod... ehm, on Factory Reset prompt: just another time don't worry! So confirm two times the "YES" option and voilà, data wiped, sorry... I'm joking, you're just in TWRP recovery!
   The recovery provided with AutoRec is obsolete (v2.8.7.0): you have to flash latest TWRP from official website.
   You can find it going here, if you own other G3 variant search your model.
   To flash the newer TWRP, simply install it from the actual recovery.
   Ps since TWRP v3.0.0 and upper is bumped by default, THERE'S NO RISK of Secure Boot error.
   
6. @rewrew asked me about KingRoot: since it will no longer be needed, how to cleanup SU binaries and any (King)junk?
   The answer is unSU.
   Flash it from recovery, so that KingRoot will be removed.
   
7. At this point... make your choice!
   
   • Strongly and personally recommended: install Magisk from the official thread.
     You'll get root permissions, also important features like Magisk Hide (to bypass SafetyNet checks) and modules support.
     Remember to update Magisk always via recovery by flashing MR_Bump.zip as second file before rebooting, otherwise you'll get the hateful Secure Boot Error.
     Ps the Secure Boot error is solvable, you have to be in condition to flash MR_Bump via recovery or bump the boot image via other methods.
     THE RIGHT MERITS: the MR_Bump zip file is developed by @dadi11 and provided by myself as "backup".
     My best thanks to developer.
     
     or...
     
   • Are you traditionalist?
     At first download the HacerPermisivo.zip, then flash it via recovery.
     Choose the official SuperSU and REMEMBER to flash the attached zip file as second file.
     Another thing to be flashed is the above mentioned MR_Bump.zip, since newer SuperSU overwrites the boot partition like Magisk.
     
     or...
     
   • Are you both traditionalist and scared by Secure Boot Error risk?
     Always download HacerPermisivo.zip and flash it via recovery.
     Now go with the system mode version of SuperSU (thanks much @alexmick for reporting that).
     BE AWARE of the purposed v2.52 beta by OP, since it's obsolete and caused issues for most users.
8. AT THIS TIME I WOULD SUGGEST YOU TO KICK OFF THE OFFICIAL FIRMWARE ("Wipe > Advanced wipe" and select all the partition, except for MicroSD and eventually Internal Storage).
   After doing that, flash the desired rom and stay happy.
   You want to stay stock? XDA developers are boring people so you don't trust in their roms?
   Okay friend, in this case you just need to clean cache... do that and finally REBOOT.
   Ps don't worry if you still see the KingRoot app installed: just remove it.

NOW YOUR DEVICE IS ROOTED PERMANENTLY (until you decide to go back to stock, by following this thread).
MISSION COMPLETED, MODDER