Creating a singleton container class for intrer-class messaging
I don't see why you need a singleton. You are already passing around container references as parameters. So, if you only create one container in __main__ and pass that to the class instances, they will all be accessing the same container.

Instead of calling Singleton() in Three.__init__ you would simply do the usual thing:

class Three:
    def __init__(self, container)
        self.container = container