First, name the problem. Start by talking about the disease of triangulation with the people that it might be affecting. Sometimes people’s intent isn’t nefarious, but they’ve found in previous relationships that talking to someone directly hasn’t worked. Now they fear it for some reason. Sometimes A and C will talk to each other about B, because where they have come from, speaking directly could have been dangerous. So tell them you’ve noticed that sometimes there’s a meeting after the meeting. At times that can be OK, if it’s to do something that is constructive.