Inthe present situation, they need to change only the second layer machine; the other two can remain the same. This is referred to as modularity.
Modularity in this case means independent layers. A layer (module) can be defined as a
black box with inputs and outputs, without concern about how inputs are changed to
outputs. If two machines provide the same outputs when given the same inputs, they
can replace each other. For example, Ann and Maria can buy the second layer machine
from two different manufacturers. As long as the two machines create the same ciphertext from the same plaintext and vice versa, they do the job.