Find (and kill) process locking port 3000 on Mac

You can try netstat
netstat -vanp tcp | grep 3000

For macOS El Capitan and newer (or if your netstat doesn’t support -p), use lsof
lsof -i tcp:3000

For Centos 7 use:
netstat -vanp –tcp | grep 3000

Find:
sudo lsof -i :3000

Kill:
kill -9