Mediator, Memento, and Observer are three common design patterns described in the Gang of Four (GoF) patterns book. The document discusses each pattern and potential questions that could arise in their implementation. For Mediator, it notes that a mediator tree may be needed to handle cooperative behavior across objects. For Memento, it discusses how to capture an object's state to allow returning to previous states while ensuring encapsulation. And for Observer, it describes how dependent objects can be notified of state changes using push/pull models with a mediator to control updates.