Does Curse of Opulence give its controller tokens when someone else attacks?
You are correct, players A and C each create a Gold token.

The ability "Whenever enchanted player is attacked, create a colorless artifact token named Gold" applies to the player that controls Curse of Opulence, which is player A in this case.

The other part of the ability, "Each opponent attacking that player does the same" makes it so that player C also gets a token.


  303.4e An Aura’s controller is separate from the enchanted object’s controller or the enchanted player; the two need not be the same.


The player who controls Curse of Opulence is also the controller of the triggered ability:


  112.8. The controller of an activated ability on the stack is the player who activated it. The controller of a triggered ability on the stack (other than a delayed triggered ability) is the player who controlled the ability’s source when it triggered [...]


And the controller of the ability is the one that carries out the instructions, including to create the token:


  608.2c The controller of the spell or ability follows its instructions in the order written.