So, I am writing the steps here with the necessary files so that everyone can find it in one place.
This is not my work. All credits go to the original developers. I am just consolidating the steps I followed here. If I am violating any rules, please feel free to close this thread.
@akhilnarang for Modified TWRP
@Chainfire For SuperSU
Oreo Compatible TWRP: https://drive.google.com/open?id=0B4...3dBbzJhMWJUVE0
SuperSU Zip: https://drive.google.com/open?id=0B4...1U3c3JSRmF2dWs
Steps to install TWRP and Root:
Note: My phone was encrypted when I tried this.
1. Copy the downloaded SuperSU Zip file to your phone.
2. Copy the TWRP file to the folder where ADB and Fastboot are installed.
3. Turn off your OnePlus 3/3T. Boot your OnePlus 3/3T into Fasboot mode > Press and hold the Volume Up Button + Power Button.
4. Connect your phone to your PC via USB Cable.
5. Open a command window on the folder where ADB and Fastboot are installed. Go to the folder where ADB and Fastboot are installed. Hold down Shift Key > Right Click on the empty space > Choose ‘Open CMD Here’.
6. Execute the below command in the command window to see if your device is properly connected:
fastboot flash recovery TWRP-Oreo-OnePlus-3-3T.img
8. Assuming you are now in Fastboot Mode, use the volume buttons and choose ‘Recovery Mode’ and then press the power button to enter TWRP Recovery.
9. Once you are in TWRP > Tap on ‘Install’ > Choose the SuperSU Zip file which you have downloaded from above > Swipe right to flash.
10. Reboot your device.
Let me know if you have any questions/doubts!