I just installed Visual Studio 16.1.0 recently. I can no longer target .NET Standard 3.0 or .NET Core 3.0 and when I go to Tools > Options > Projects and Solutions > .NET Core there is no checkbox to enable preview releases.
I do have .NET Core 3.0.100-preview 4 installed as well
I have repaired Visual Studio, uninstall/install, and for some reason, I have no option to enable preview releases. I'm hoping that this will also solve my TFS build issue since my machine is a build agent.