It is easy to know that the call to the main timeline is _root.
No matter where it is called, it can start from the main timeline.
No matter where A calls C, it depends on which movie clip C is on the main timeline.
As you said, to call C, use it when you need to call it.
_root.c.play()
Do it.
Of course, if your B and C are on the main time axis, then A in B can also call C in this way on the A button:
Open (press) {
_parent.c.play()
}