/Users/Username/.rvm/gems/ruby-2.3.1/gems/puma-3.4.0/lib/puma/binder.rb:255:in `initialize': Address already in use - bind(2) for "0.0.0.0" port 3000 (Errno::EADDRINUSE)

To see what's running on port 3000, type the following into the terminal: lsof -i tcp:3000. It should show you the open file or process on that port. To kill that process (so you can free up port 3000 for your Rails server), copy the PID number and then type this into your terminal: kill -QUIT <PID> (remember to replace <PID> with the number you copied before)