What I meant was, that Person C should wait for Person A to break up - and if Person C is really keen on Person A, they should be asking Person A to break up. There's nothing wrong with pursuing person A as long as person A is reciprocating (if person A is not interested then person C should respect that and leave them alone).
Personally, if ever I was in that situation, I'd let person A know that I liked them. And if they become single great! If they break up as a result of me telling that to person A, I'd be a little upset for Person A, but at least I did it the right way. I wouldn't want to be with Person A if I knew they were still committed.
Now if person A is aware of C but is in an open relationship, then of course it is a different dynamic. I wouldn't/couldn't be in that sort of relationship, but at least everyone is open and honest with one another.
The rub for me is the lack of honesty. Sometimes truth hurts, but in the long term it works out for the best.