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You do not mention whether you're telling your story in first person or third, and if in third person - limited or omniscient. In third, your task is easier, since you have the narrator to mention things that C might be missing.

I do not mean the narrator hanging a lampshade: "C didn't notice how A was gesturing to B behind C's back". That would usually be too much. But the narrator can well mention A and B exchanging glances, fidgeting, considering their words (this last one is better suited for an omniscient narrator since that's an intrusion into A and B's heads). You'd be using the characters' body language to convey their dishonesty. It's not *what* they're saying, but *how* they're saying it.

If you're telling the story in first person, letting the reader know something the character doesn't might be counter-productive. Telling a story in first person serves to bring us very close to the character's experience. By letting us know something the character doesn't, you're creating distance. In this situation I think I would let character C notice A and B are uncomfortable for some reason - this could come from how they act and how they interact. But I would give some alternative explanation, some distraction, as to why they might be uncomfortable. Then, when the true reason is uncovered, everything will fall into place.

What A and B are saying can also be vague and circumspect, but there's a problem there: if they're saying nothing, their words are just noise, you're boring the reader. They need to be saying *something*. It's just that they're not saying *all*. So what *are* saying needs to be sufficiently interesting, needs to propel the story forwards.

(I'll try to come up with examples later.)