When you deploy the oneagent in the app service this will be deployed on the server where it runs detecting the appservices on it. if the 3 apps service are sharing the same VM for the service plan. this will only detect the 8gb because only 1 agent will need to be deployed. as far, this have work for me in this way.
some times an app service 1gb and 1.75gb create a service plan by each one so here every app service consume a 0.25 host units.
even in the demo you can see 2 app service in the same server.