To each their own. With a friendship triad, we have a 'everything you tell me, the other one will eventually hear, too' agreement. I'm not a hugely private person and tend to stick to the rule 'if there is something I don't want someone to know, I won't tell it to anyone'. In practise I find it exhausting to try to remember what was okay to repeat and what wasn't, so I try to tell people not to share something with me that is truly off-bounds from being told to anyone else.
As to gossip, if it's one-way only, that's potentially an unhealthy sign for me. I mean, you and B keep on talking about the personal life details of A, but you never share any of B's stuff with A (this is a rhetorical question, I honestly got confused with the letters :P). That would border on gossip, for me.
"Resentment destroys more alcoholics than anything else. From it stems the root of our spiritual disease."
"In dealing with resentments, we set them on paper - list people, institutions and principles with whom you are angry. Ask yourself why you are angry."
"In most cases it was found out that our self-esteem, our pocketbooks, our ambitions, our personal relationships, including sex, were hurt or threatened. We were sore, burnt-up." Alcoholics Anonymous, 64-65.