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The way you do this effectively is through dramatic irony, which is where the reader knows something that the protagonist does not know. There is a fantastic scene in Upstairs Downstairs where one of the characters is seeing another off on an ocean voyage and they are all happy and full of plans and triumphant optimism and as the scene proceeds, the camera slowly zooms out and you see the name of the ship: "Titanic". That's dramatic irony. I don't remember which characters it was or what they were happy about but more than 30 years later I still remember that zoom shot. 

In the case of a conversation like this, the way you use dramatic irony is you make sure that the reader is already aware of whatever it is C does not know before the conversation begins. Then the reader perceives at once that C does not know the secret and that A and B are deliberately avoiding mentioning it. The scene itself requires no special technique. It is all about how you set it up. 

This should be taken as a general rule. To create an effect in a scene, set it up earlier. Don't try to do it by manipulating the way you tell the scene itself. That kind of thing almost always sounds contrived and takes the reader out of the scene. The craft is in the storytelling, not the prose.