If so, link to API server should be "myapp.com:3000", right?
If you want to call to API server as a path - such as "myapp.com/api/" - you may need an adapter for your front-end. It's a small script using Node.js or nginx configuration that will map and redirect every request to artual target (at port 3000).
For my opinion, the better approach is using sub domain. I often apply this method: map the domain "api.myapp.com" to port 3000 so that I can send AJAX requests to there. Why this is good? Sometimes you may need to move API to another server. Sometimes your API may run on many clustering servers, you just need to map api.myapp.com to an HAProxy for example.
Frontend Developer