There can be only one web server listening on a given port. You can directly "listen to a path", only a port.

So, if you already have an existing web server running on ports 80 and 443, then these are your options.

1. Modify your existing web server to add your API server code to it so it can handle requests for http://yourdomain.com/page3 and https://yourdomain.com/page3 directly with your API.
2. You can add some middleware to your existing web server to make it a proxy for http://yourdomain.com/page3 that will redirect requests to your http://yourdomain.com:3000/page3 Express-based API server.
3. Run your own Express-based API server on your own separate port and access the API server directly via the separate port such as http://yourdomain.com:3000/page3.
4. You can install a proxy such as nginx in front of your existing web server and have it redirect incoming requests to http://yourdomain.com/page3 and https://yourdomain.com/page3 to the separate port that your Express-based API server is running on.