The default port is determined by PORT environment variable so you can put that next to your start command. For example, you can change react-scripts starts in your package.json to PORT=8000 npm start (or set PORT=8000&npm start on Windows).

Oh, big thanks. I think it would be better if this is mentioned in the readme.

Where is 3000 set in the source code? I need to be able to set that to an environment variable.

I need to be able to set that to an environment variable.

You can already set the PORT environment variable to anything and it will work.
3000 is just the default.

I think it would be better if this is mentioned in the readme.

I agree, we plan to document this soon.