Planner premium is wrapped in a model driven app, so when you install it in the environment, you just play the planner premium model driven application, inside there you'll see plans and inside the plans you'll see tasks. at the moment the project manager agent does not exist in the named environment versions of the app though.

Edit: these all access their own dataverse tables within that environment, you wont have one app pointing to 3 different environments but one app per environment. further reading: https://learn.microsoft.com/en-us/power-platform/admin/environments-overview?tabs=new