//If you don't want to set the environment variable, another option is to modify the scripts part of package.json from:
"start": "react-scripts start"
//to
//Linux (tested on Ubuntu 14.04/16.04) and MacOS (tested by aswin-s on MacOS Sierra 10.12.4):
"start": "PORT=3006 react-scripts start"
//or (may be) more general solution by IsaacPak
"start": "export PORT=3006 react-scripts start"
//Windows
"start": "set PORT=3006 && react-scripts start"