We can build windows application with WinForms and WPF on .Net Core 3.0
1. Create your first WinForms app on .NET Core
https://channel9.msdn.com/Series/Desktop-and-NET-Core-101/Create-your-first-WinForms-app-on-NET-Core
2. Create your first WPF app on .NET Core
https://channel9.msdn.com/Series/Desktop-and-NET-Core-101/Create-your-first-WPF-app-on-NET-Core