sorry I may have given the hours and minutes in reversed order...
Try:
30 03 * * * /usr/bin/certbot renew --nginx
DOH!
30 is a bad hour indeed!
If you don't care too much about possible nginx restarts (when cert renews - every 60 days), just use:
30 03 * * * /usr/bin/certbot renew --nginx -q --deploy-hook 'service nginx restart'