iptables -A INPUT -m tcp -p tcp --dport 3000 -j DROP
а лучше так
iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
iptables -A INPUT -p icmp -j ACCEPT
iptables -A INPUT -i lo -j ACCEPT
iptables -A INPUT -m state --state NEW -m tcp -p tcp --dport 22 -j ACCEPT
iptables -A INPUT -m state --state NEW -m tcp -p tcp --dport 80 -j ACCEPT
iptables -P INPUT DROP