I had replied to your same question in another thread. I’m moving it here to avoid duplicates. 
That’s an error to be corrected in the docs, see TWRP in install instructions for the FP3?
Just skip the entire “Installing a custom recovery” section of https://doc.e.foundation/devices/FP3/install
I personally would go with the official instructions. The other topic (how to install on Debian based linux) might help a bit, because it contains a couple of screenshots. But essentially, the actual things to be done are the same. If there are differences, I’d go for the official docs, as other tutorials might not always adapt the latest changes.