Frameworks which provide Behavior<T> in .NET Core 3 / WPF

This answer describes in details what needs to be done: https://stackoverflow.com/a/56240223/532575 .

In short, the Microsoft.Xaml.Behaviors.Wpf NuGet package is a replacement for Microsoft.Expression.Interactions and System.Windows.Interactivity.

You can use the .NET Framework version of the WPF Behaviors in your .NET Core 3 project as is - it is 100% compatible (the build warning is ignorable).