In my experience, yes, A will defend against C like it has against B. This means that it is not favorable to attack a stack of 3 infantery units with 5 stacks of 1 infantery unit. It's better to combine your 5 stacks of 1 infantery to 1 stack of 5 infantery and use that to attack.
Most units solve their attack rounds every hour. An exeption are planes off course, not sure about some ships and artillery.
