The HelloMobile project demonstrates a working example referencing the same multi-targeted ServiceModel, Server.Common and Shared.Client Gateway projects referenced in 4 different AppHosts AppHosts:
I recommend starting from an empty working project, then slowly copy everything you want into the working project so you can identify when something causes it to break.
The .Core projects only contain .NET Standard 2.0 builds which can only be used in ASP.NET Core projects running on the .NET Framework and .NET Core platforms.
The only other solution is to copy the source code into the different projects in order to decouple any binary dependencies.