Find (and kill) process locking port 3000 on Mac and Linux

1. You can try netstat

netstat -vanp tcp | grep 3000

2. For macOS El Capitan and newer (or if your netstat doesn't support -p), use lsof

lsof -i tcp:3000 

3. For Centos 7 use:

netstat -vanp --tcp | grep 3000

kill {pid_of_process}