Assuming support for the encryption used on the Fairphone 3, as it comes encrypted by default.
The track record of TWRP regarding encryption as of late doesn’t suggest that, and encryption support is not a guaranteed feature of TWRP.
If it works like on the Fairphone 2 and only user data is encrypted and the OS is not, TWRP would still be useful to backup the state of the OS.
Why not?
On the Fairphone 2 TWRP saved backups in a directory named after the phone’s ID.
If you wanted to transfer the backup to another phone and restore it there, you just had to rename the directory after the other phone’s ID and TWRP would happily restore this.
But, of course, until TWRP is available for the Fairphone 3, everything will be speculation.