Just to note something.

Make sure that you use process ID, not a port.
For example if you run kill -9 3000, it’ll kill process with ID 3000, not an app running on that port.
Execute following command:

sudo netstat -lpn | grep :3000

It should return app with its ID. Use kill command on it

sudo kill -9 id

`-9 means that process will be forced killed.