protocol MyProtocol {
    func doTheFirstThing()
    func doTheSecondThing()
    func doTheThirdThing()
}

extension MyProtocol {
    func doTheFirstThing() {
        print("The DEFAULT first thing")
    }
    
    func doTheSecondThing() {
        print("The DEFAULT second thing")
    }
    
    func doTheThirdThing() {
        print("The DEFAULT third thing")
    }
}

class MyClass: MyProtocol {
    func myMethod() {
        self.doTheFirstThing()
        self.doTheSecondThing()
        self.doTheThirdThing()
    }
    
    func doTheFirstThing() {
        print("This implementation of the first thing should be called instead of the default implementation")
    }
}

class MySubclass: MyClass {
    
    func doTheSecondThing() {
        print("This implementation of the second thing should be called instead of the default implementation")
    }
}

MySubclass().myMethod()
// This implementation of the first thing should be called instead of the default implementation
// The DEFAULT second thing (This should be the implementation from the subclass)
// The DEFAULT third thing