Combining multiple services in one at the level of ArcGIS Server,
Suppose that we have 3 municipalities that collects the data of buildings and each municipality creates a service for its buildings layer. Then how these three services can be combined in one service such this service can be consumed as “one unit” in a web mapping application?
Depends on the web platform you are using. I don't think it possible in WAB and AGO, but if you roll your own custom JS API based app, you can most certainly customize the layer click button to toggle all three layers at once.
I think the issue is layer permissions for different user roles that allow editing capabilities for certain users but read-only for all other. You can set up these roles at service level but not layer level within a MXD.