Prerequisites:

1) The first and foremost thing to do is rooting your LG smartphone. You can follow the tutorials given below to do it:

a) How To Root LG G3 (One-Click Root Method)
b) How To Root LG G3 (D855/D851/D850) Android Phone

2) Next, enable USB Debugging mode in your smartphone.

3) Deactivate or uninstall antivirus applications in both your device as well as in your computer as they tend to disrupt the installation process.

4) Download LG USB drivers for your smartphone and install it in your computer. By doing so, you will be able to connect your device to the computer successfully.

Upon finishing with these prerequisites, you can move to the main installation tutorial.

Tutorial To Install TWRP Recovery In LG G3 Smartphone:

1) To get started, download TWRP recovery files given below based on your variant to your computer.

a) TWRP Recovery for T-Mobile LG G3 D851
b) TWRP Recovery for AT&T LG G3 D850
c) TWRP Recovery for Sprint LG G3 LS990
d) TWRP Recovery for Verizon LG G3 VS985
e) TWRP Recovery for LG G3 Korean LG G3 F400K/S/L
f) TWRP Recovery for LG G3 D852
g) TWRP Recovery for LG G3 D852G
h) TWRP Recovery for LG G3 D855
i) TWRP Recovery for U.S. Cellular LG G3 US990

2) Next, connect your LG device to the computer with the help of original USB data cable and transfer the TWRP recovery image file to your SD card.

3) Now, download and install Flashify app from Google Play store in your device. Next open the application. While doing so, the SuperUser app will ask you to grant permissions for running Flashify. Grant it.

4) In the app screen, select Recovery image -> Choose a file and browse to the SD card folder where you have saved the TWRP image file. You will see a prompt asking for your permission. Just click on “OK” to start the flashing process and once done, select “Reboot Now” in the next screen.

That’s it. Now you have successfully installed TWRP recovery on your LG G3 smartphone and now you can install MODs, custom ROM firmwares, etc., of your choice. To access TWRP recovery, download and install QuickBoot app from Play Store

If you encounter any issues while following this tutorial, do let me know via comments.