Not that sure I can nut that out at the moment, I'm a bit tired.
Sounds like you have A and C sussed, the relevant question is does person B share DNA on the same segment as the other two, that is, is there a triangulated match?
If not the answer is probably no, or at least there are more possibilities.
The reason for saying high end of the range is that the further you go in 3rd, 4th, 5th cousins, the greater the overlap in ranges appears to be, a 3rd cousin range can start at 0cM (I have one at 10.7 with her cross matches to other 3C in the expected range), if you have matches at the high end of the range that makes more distant relationships unlikely.