[Oneplus 3] Ultimate Guide

First get all the information you want to get done to your done as this is just a simplified thread on how to do, not on explaining so get your mind set at one place and follow the instructions.

1. Unlocking the Bootloader

This comes first as you cannot do anything to any Android Device before unlocking its bootloader. And Beware as it Wipes all Data!!!

Instructions:

• Back up the Data (Optional)
• Turn off the Device
• Connect Device to PC
• Download Fastboot Tools from here - Link
• Then open cmd in the Fastboot Tools folder
• In cmd run command to check device detection

fastboot devices 

• Then if detected run command( Going to wipe Data!!!!)

fastboot oem unlock

•  Then run 

fastboot reboot

• Done! 

 2. Flashing TWRP

• Boot to Fastboot Mode
• Connect Device to PC
• Flash the Recovery.img through Fastboot mode. - Link

fastboot flash recovery recovery.img 

•  Done!

 3. Making TWRP Backup

• Boot to TWRP
• Go to Backup->Select all->Thats all
• Now Swipe to Backup 
• Done!

 4. Rooting

• Boot to TWRP
• Go to Install->Select Magisk-v18.1.zip ->Flash Link
• Reboot
• Done!

5. Restoring TWRP Backup

• Boot to TWRP
• Go to Restore->Select the Backup you want to Restore->Swipe to Restore
• Done!

Download

• TWRP - Link
• Magisk - Link

Source - Link