You should be able to just unzip it, cd into the key/bin directory, and execute ./key. I have a Raspberry Pi 4, but I assume it should work on the 3 as well. Getting MIDI to work on the Raspberry Pi can be done by following the instructions on this page:
http://tedfelix.com/linux/linux-midi.html You don't necessarily need to use a low-latency kernel or realtime audio group.
I've also recently made the changes necessary for KeyKit to run on 64-bit machines.
...Tim...