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