TWRP Recovery is now available for One Plus 3T . It is built by jcadduono and it was shared in XDA forum.
So I am providing you the method to flash this TWRP . You can flash this TWRP by Fastboot method only because the bootloader needs to be unlocked to flash this on One Plus 3T. Read the Instructions Carefully.
WARNING: The OnePlus 3T stock boot image has dm-verity, which prevents you from booting if you swipe to enable system modifications!
SOLUTION: Follow the full rooting steps below, in order, to allow system modifications without the risk of a boot loop!
One Plus 3T
TWRP Version • 3.0.2-0
Delveloper • jcadduono
How To Flash
• Extract and install adb & fastboot to your computer.
• Download TWRP for OnePlus 3T
• On your phone, go to Settings > About device. Tap 7 times on Build number to enable Developer options.
• Now go to Settings > Developer options. (above About device)
You need to enable OEM unlock , otherwise you might end up with boot problems or be unable to flash later on.
• Reboot your device into fastboot mode. To do this, select reboot from the power menu and hold the [Volume Up] + [Power] buttons while your device reboots.
• Once you're in fastboot mode, you need to unlock your bootloader. To do this, run this command:
fastboot oem unlock
WARNING: Unlocking your bootloader will erase all userdata on the device! Your device will be completely reset to factory defaults!
• Once your bootloader is unlocked, you're ready to flash TWRP. To do this, run these commands in order:
fastboot flash recovery twrp.img
fastboot boot twrp.img
• At this point, you will reach the screen asking you if you want to allow system modifications.
By swiping right, you will trigger dm-verity, and if you don't follow the next step you will be unable to boot!
If you are going to root your device and follow the rest of these steps, then it is safe to swipe right and enable modifications.
• Root with SuperSU
Download the latest SuperSU by Chainfire.
Without exiting TWRP, transfer the SuperSU zip to your device over MTP and flash it using Install in TWRP.
•Go to Reboot > System
• Wait 2-5 minutes for your device to finish setting itself up. (SuperSU will reboot you a few times)
Its Done Now you can use recovery to flash ROMs , Mods and other flashable zips.