Kill the process that is using a port

Something is already listening on the port your dev server wants. Find it:

sudo lsof -i :3000

Or with ss, which is installed everywhere these days:

ss -tulpn | grep :3000

Both print the PID. Kill it politely first, then not so politely if it ignores you:

kill 12345
kill -9 12345

If you don't care who it is, fuser does the lookup and the killing in one go:

fuser -k 3000/tcp