How can I create a Java Project that involves a combination of 3 design patterns? (Namely, Singleton, Abstract-Factory and Observer)

So, I am to create a Hotel Booking System, by using Singleton, Abstract-Factory and Observer-Pattern? What I want to know is how exactly do I combine all three in order for the Front Desk Agent can carry out the check-in and Check-out procedures? I do not need code but I need to know how I could develop a UML using all three patterns? Right now I have 20 classes, out of which 4 are interfaces.