Success !! Thanks @Lakotaubp and @NoelFlantier

Here is a short summary of the actions I made:

• On PC: - Install "UBPorts" and "Minimal ADB and Fastboot"
  - Download TWRP recovery image and put it in "ADB and fastboot" folder (in program files for Windows)

• On phone: - Change to developer settings, activate debugging USB and OEM unlock
  - Download SuperSU zip file from chainfire

• On PC: in a terminal enter:

fastboot flash recovery twrp.img

Then

fastboot boot twrp.img

• On phone: When on bootloader plug your phone to PC's USB so TWRP Recovery will start
  Click on Install
  Look for superSU.zip file on your phone
  Slide the arrow to the right
  Phone will reboot at least 2 times

• On PC ; use again the fastboot boot twrp.img to restart TWRP
  *On Phone: Click wipe
  Then Repair and change format to ext4 for "System" "data" and "cache" folders
  Then back to wipe menu select the 3 folders and slide the arrow to the right

• On PC: Launch UBPorts and install Ubuntu Touch

• On Phone: When you reach the Android recovery screen, go to Android data> android actions > Full factory reset

• On PC: launch installation again if needed

Then your done !

Helpful pages with download links:
https://forum.xda-developers.com/oneplus-3/oneplus-3--3t-cross-device-development/recovery-official-twrp-oneplus-3-3t-t3543391
https://forums.ubports.com/topic/3253/oneplus-3-3t/391

Hi @CAnoop,

The only thing I think about regarding your question is that one of your folder has not been properly converted in ext4
Make sure you try them 1 by 1 using "Repair" or "Change" functions
If you have an error message saying you can't convert, then try using other functions of TWRP, you should be able to convert them all 3.

Also, According to what I read in this forum, the "Android actions-> Full Factory Reset -> Wipe data" action is not always necessary.

Good luck !

Success !! Thanks @Lakotaubp and @NoelFlantier

Here is a short summary of the actions I made:

• On PC: - Install "UBPorts" and "Minimal ADB and Fastboot"
  - Download TWRP recovery image and put it in "ADB and fastboot" folder (in program files for Windows)

• On phone: - Change to developer settings, activate debugging USB and OEM unlock
  - Download SuperSU zip file from chainfire

• On PC: in a terminal enter:

fastboot flash recovery twrp.img

Then

fastboot boot twrp.img

• On phone: When on bootloader plug your phone to PC's USB so TWRP Recovery will start
  Click on Install
  Look for superSU.zip file on your phone
  Slide the arrow to the right
  Phone will reboot at least 2 times

• On PC ; use again the fastboot boot twrp.img to restart TWRP
  *On Phone: Click wipe
  Then Repair and change format to ext4 for "System" "data" and "cache" folders
  Then back to wipe menu select the 3 folders and slide the arrow to the right

• On PC: Launch UBPorts and install Ubuntu Touch

• On Phone: When you reach the Android recovery screen, go to Android data> android actions > Full factory reset

• On PC: launch installation again if needed

Then your done !

Helpful pages with download links:
https://forum.xda-developers.com/oneplus-3/oneplus-3--3t-cross-device-development/recovery-official-twrp-oneplus-3-3t-t3543391
https://forums.ubports.com/topic/3253/oneplus-3-3t/391

Hi @Lakotaubp ,

I managed to install TWRP recovery but don't understand the next step about clearing cache format partition to ext4...
I'm stuck there for now, digging,
any clue is welcome

Hi @NoelFlantier

I've manage to go through all steps.
What am I supposed to do after that ?
Root the phone ? Not sure what/how to do that

Thx

@Lakotaubp
Thanks for helping, however I'm new to this and I'm not sure to understand all the meaning of this thread.

I'm using TWRP App (see screen shot) and it looks quite different from what I've seen online
WHen I open it, it says: "device not rooted"
Then I still have the same screen as described before (see screenshot 2)

@NoelFlantier , Thank you, but I'm stuck at stage 7.

Terminal tells me: error: cannot load 'twrp.img': No such file or directory
I've name my file twrp.img but don't know where to put it so the system will find it...
Any clue ?

I have dual boot on my laptop but tried on both OS: Ubuntu20 and Win10

Hi,

After checking the different options on this forum to solve my issue, I finally come to ask for help to find what I've missed...

Pb1: with TWRP (https://twrp.me/oneplus/oneplusthree.html)
After downloading image file with 'TWRP flash' function on my phone using TWRP official, the app can't find the file location when trying to 'backup existing recovery'

Pb2: Using UBports, every time I try to install Ubuntu touch, the software says: Device connection lost and the screen is blocked on: "Preparing System Image"
And the smartphone displays: the 'Android Recovery' page

I followed below procedures:
https://forums.ubports.com/topic/3636/oneplus-3t-installation-with-ubports-installer-0-4-14-beta
https://forums.ubports.com/topic/3985/problems-installing-ut-on-oneplus-3t/2

Thank you for your help !