These are the instructions to root LG G3 (F400K, F400L, F400S, LS990) using IOroot Kit on Windows Computer.
Requirements: Your LG G3 smartphone should have at-least 20-30 percent of battery to perform the rooting process.
Step 1: Download and install LG G3 Driver on your Windows computer. In case LG G3 Driver is already installed then skip this step.
Step 2: Enable USB Debugging on your smartphone. To Enable: Settings > Developer Options > USB Debugging.
Step 3: Download and extract the LG G3 root files on your computer. After extracting you will be able to see the following files:
Step 4: Now, Open root.bat from the extracted files and connect your smartphone to the computer.
Step 5: Once root.bat is launched press any key to begin the rooting process.
Step 6: Now, it will ask you to unplug the smartphone from the computer and disable the USB debugging and re-enable it. Once you are done connect your smartphone to the computer and press any key to continue the root process.
Step 7: Now, it will give you final warning message. Simply press any key from the keyboard to continue.
Step 8: Now, disconnect your smartphone from the computer and restart it.
Step 9: Now your LG G3 is Rooted. To check whether your phone is rooted correctly, open Applications Menu, there you will be able to see a new app installed, called SuperSU. If this app exist then it means you have successfully rooted your device.
[*] No data loss process: Your Phone Data like contacts, pictures, videos, music and etc will be not affected with this method.
[*] Warranty Void: Your LG G3 Warranty will be Void after following the above tutorial.
[*] LG USB Driver: If you are looking for the original USB Driver for your device then head over to Download LG USB Driver for all devices page.
[*] Credits: Thanks to thecubed (xda developer) for providing the root files. Without using the root files the above tutorial was not possible to made.