[G3 Q&A, Help & Troubleshooting] dksoni411:
Sometimes it might have happened to you that you have erased your download mode while doing some fun with adb commands and enjoying android development with starter knowledge. In this process you unfortunately destroyed your download mode.

Now you want to go back to stock but your download mode is not appearing only a fastboot mode opening saying some very tensed and confusing things to understands. This is the situation where you are totally messed up and out of mind. Don't know what to do. Tried whole of internet and can't find a solution. Now method is working and you want a fix. Then this thread is gonna help you a lot. You will be very happy to read instructions here cause they are easy and also phone saving too. These instructions can even save your amount to be given to a repairing shop.

So a lot of introduction is done, let's proceed to the process. First of all read disclaimer to avoid any misunderstandings of future.

Your warranty is void now. I am not responsible for any mistake and wrong thing happens to your device. Always try if you have a good knowledge of that.
These steps are not an official LG and therefore not endorsed by LG Electronics in any way. As such, this is a 'try-at-your-own-risk' approach. Please make sure to back up your important files when trying this for the first time. I take no responsibility for any undue outcomes resulting from the use of these steps and instructions. And all the best for step.

Things you Need:

1: LG united mobile drivers here. (Skip you have already)

2: adb & Fastboot drivers from here. (To do flashing commands). Minimal Edition.

3: Make sure "USB debugging" is on for you LG G3 device. If not sure check here.

4: laf.img for your device . (Download mode image). Most important thing

Download it from here according to your device variant like as d850, d855 or etc.

Select your variant > Stock partitions > 10D > laf.img ( I prefer 10d laf.img you are welcome to try other ).

5: LG G3 device in fastboot mode by connecting to PC while phone is powered off pressing volume up(+) button.

6: And the most important thing a computer. (I tried it on Windows 10 but will work on 7,8 and others too.

Steps:

1: Install LG united mobile drivers.

2: Install "adb and fastboot mode" drivers. Copy "laf.img" to the directory where you installed them and run "MAF32.exe"

Mine is "C:\Program Files (x86)\Minimal ADB and Fastboot"

3: Connect your LG G3 device while in fastboot mode.(Make sure drivers are installed).

4: Check if connection is right by doing this command

fastboot devices

5: If you done right your device might appear. (Sometimes as a question mark too)

6: To flash download mode. Execute these commands in sequence.(Make sure "laf.img" is copied to that folder where you installed the adb and fastboot drivers and is named as "laf.img").

fastboot erase laf
fastboot flash laf laf.img
fastboot reboot

7: Pull battery of your device and reboot into download mode again if flashing is successful. Download mode might appear as original. If not then you might have not listened to instructions. (If still stuck ask in comments).

If i helped you anyhow press "Thanks" :)

All the best with process.

Credits:

Autoprime for the stock partitions.

shimp208 for minimal ADB and fastboot drivers.

And all others who gave some ideas, drivers and tools for that method to work.