Generally for most applications that provide their own http service you would configure a web server like nginx on your droplet to act as a proxy. This would mean that nginx would accept requests on port 80 (http) or port 443 (https) or both and route those requests to the service running on port 3000, returning the data the same way.

To do this, first install nginx:

apt-get update
apt-get -y install nginx

Then open the file /etc/nginx/sites-enabled/default and edit it to look like this:

server {
        listen   80; 

       location / {

        proxy_set_header X-Real-IP  $remote_addr;
        proxy_set_header X-Forwarded-For $remote_addr;
        proxy_set_header Host $host;
        proxy_pass http://127.0.0.1:3000;

         }
}

and restart nginx for the change to take effect:

service nginx restart

This should get your site to respond on port 80. I would recommend, once this is configured, you look into configuring a firewall to block public access to port 3000.

It works thank you!