Say I have the following scenario:
3 different dommains
If you don't have a strict restriction on creating a new web app. I suggest you create new web apps for WebSiteB, WebSiteC, and they share the same app service plan with WebSiteA, because all web apps inside the same service plan share the resources of the plan. Let's say you will have
myservera.azurewebsites.net myserverb.azurewebsites.net myserverc.azurewebsites.net
Next, you can create CNAME record to map your custom domains to each web app.
I hope this helps.