Each of the three brothers was chained to one of the seven pillars, so that even though they shared a dungeon, they were isolated from each other. They weren't able to move at all, not even to take a single step.
They couldn't even see each other's faces properly because it was so dim in the dungeon.
In the gloom, his brothers' faces looked like strangers to him.
The three brothers are imprisoned together, and yet separately.
Their hands are chained ("fetter'd"), but their hearts are still "joined" together. But at least they had the comfort of being able to talk to each other, even in the absence ("dearth") of fresh air, light, and water (the "pure elements of earth").
The brothers take turns comforting each other – they offer each other hope, and sometimes share stories or legends about brave heroes. But, after a while, even those stories and legends stop comforting them.
The speaker says that, after a while, their voices started sounding "dreary" and sad in the echoes of the dungeon.
Even their voices start sounded imprisoned – they take on a "grating sound," and don't sound as "full and free" as they did before they were thrown in the prison.
The speaker admits that he could be imagining this, but to him, their voices didn't sound right, somehow, when they were in the dungeon.