Generic type parameter in .net(C#) whose value can by one of specified types?-Collection of common programming errors

Lets say I have classes ClassA, ClassB, ClassC. All of them inherit from object and non of them implements any interface.

Is it possible in C# to declare generic interface that will take generic argument T and that T may be either ClassA or ClassB or ClassC ?

So I would have something like

public interface MyInterface where T: ClassA | ClassB | ClassC {
    ...
}

If it is possible what’s the syntax?

Note: I know I could have those classes ClassA, ClassB, ClassC to implement some interface and than use it as constrain of T but before I create an interface that have no methods I want to know that there is no better way.