I don’t know how to do that, and luckily you don’t need to do that, because …
… there simply is no recovery partition on the Fairphone 3. You can’t flash TWRP to the Fairphone 3 this way.
Installing TWRP would work differently, but this doesn’t help now.
This should work with an unlocked bootloader.
What happens on the phone when it should be booting TWRP?
Apart from the warning screen displayed due to the unlocked bootloader, there could be a second screen coming up, where you would have to press the power button to proceed, else the phone would reboot after 30 seconds or something like this.