You are linking to line 29 “recovery_boot” … for booting into recovery “<kbd>Volume Up</kbd> + <kbd>Power</kbd>” is totally correct. It just isn’t needed to install /e/ on the Fairphone 3, the TWRP reference currently found in the install guide is an error, to be removed.
Notice how line 14 “download_boot” says " <kbd>Volume Down</kbd> + <kbd>Power</kbd>" … which is correct for booting into Fastboot Mode aka bootloader.
Press Volume + or Volume - to display boot options, select fastboot and confirm with Power to reboot on fastboot
When the warning is displayed, I can pause the boot, and then resume it. Then it tries to boot and fails. I can then start it again, or factory reset. If I try “Volume +” or “Volume -” in the warning state the device powers off. In the other menu, fastboot is not an option.
The only way I found to access fastboot is “Volume -” + “Power” from the powered down state.
[EDIT] It is indeed the case. The string assosciated with download_boot is nowhere to be found in the FP3 manual.
A search for "With the device powered off, hold <kbd>Volume Down</kbd> + <kbd>Power</kbd>. Release when boot logo appears." only finds https://doc.e.foundation/devices/FP3 which is not a page that you look up during installation.
Did you have updated to Android 10 before you tried to install /e/?
I think the boot sequence/behaviour with unlocked bootloader differs a bit, i.e. my guess is the description only matches 100% when coming from Android 9 stock rom.