Re:Question regarding duchy ownership change
The rules I have state:
"The game ends when at the end of a turn any one player has no more knights available. If the player who used all his knights obtains possession of further castle landmarks on the final turn, the player uses additional knight tiles to mark his possessions."
Note that "turn" is defined as a single players turn (and not a "round" in which every player gets a turn) so in your example the game would end without B & C even getting a final turn.