On Unix-based systems, the default HTTP port 80 is only available to program (executable) started by the root user.
For security reasons, it is not desirable to run the server as root (if the program got hacked for instance).
This page shows you two options on how to give access to a program without starting it as root.
The first option is to give permission (capabilities) to the executable via setcap
sudo setcap 'cap_net_bind_service=+ep' /path/to/your/executable
# To delete the permission
sudo setcap -r /path/to/your/executable
Or you can redirect the port. If you're program is listening on the port 3000, you can redirect it with the following iptables command
sudo iptables -t nat -A PREROUTING -p tcp --dport 80 -j REDIRECT --to-port 3000