Hi,
I had pretty much the same problem when trying to flash an FP3 with /e/OS just a few days ago. It turned out that the Android Bootloader Interface and the Android Composite ADB Interface drivers on my Win11 PC where not the appropriate ones, even though I had followed the respective /e/OS installation instructions. For me, the procedure described here: https://forum.fairphone.com/t/fastboot-flashing-unlock-waiting-for-any-device/118071/6 and here: https://support.fairphone.com/hc/article_attachments/18494377825809 solved the problem. In short: get the appropriate drivers by searching optional driver updates with Windows update while the phone is connected in fastboot mode. I hope this will work for you as well. Crossing fingers.