I'm having trouble building with this. Can you elaborate a little bit more about the steps after adding adding all three project. I can't understand this

>> " Now let's connect our databases to our app by adding their "keys" to our app's environment ("env") files. You should generally keep connection keys like this secret, so first we'll rename our environment files to keep them from getting committed into git (the new filenames are listed in .gitignore). "

My fault--I've re-added the sample env files, you can copy these, remove ".example" from the name, and add your keys:

https://github.com/sampl/firefly

Thanks for trying it out!