Can WPF applications be run in Linux or Mac with .Net Core 3?
Microsoft announced .NET Core 3 comes with WPF and Windows Forms. So can I create a desktop application for Linux or Mac using .NET Core 3?

No, they have clearly stated that these are windows only. In one of the .NET Core 3.0 discussions, they have also clarified that they do not intend to make these features cross-platform in the future since the whole concept is derived from windows specific features. They talked about thinking of a whole new idea for cross-platform applications, which is not easy. 
Source: https://youtu.be/HNLZQeu05BY
Update
The newly announced .NET 5 now aims in avoiding all this confusion by no longer calling it ".NET Core".
Update 2
With blazor client-side (releases on may, 2020), there is a new experimental project for cross-platform apps using webview that is in the works. 
Source:
https://blog.stevensanderson.com/2019/11/01/exploring-lighter-alternatives-to-electron-for-hosting-a-blazor-desktop-app/
