Please correct me if I am wrong but it looks as if you are working from the excellent guide [HOW-TO] Flash /e/-OS on Fairphone 3 using Debian based GNU/Linux which was sadly last updated in June 2021.
On this current page https://doc.e.foundation/devices/FP3/install the fastboot flashing of the various downloaded .img files is carried out by
./flash_FP3_factory.sh