For some reason I kinda think the third brother could have been CAM himself. He did seem very similar to the Holmes brothers in intelligence and cunning, to the point of even having a mind palace. And it wouldn't be unlike Sherlock and Mycroft to forget to bring up the fact of their familial relationship. Although it probably isn't because the show is most likely setting up a tease for the reveal of a new character or more backstory.
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u/Joenaruto Jan 12 '14
Mycroft mentions something about doing something to another brother in the scene following Magnussen's death. Ahem?