From that last picture I would almost say that looks like caterpillar munching damage - the black specs could be poop, unless those specs are tiny critters. I had not ever seen caterpillar damage on a cactus until this late summer/early fall when I found a few caterpillars munching away at the tip of a cactus in my back yard.
The damage in the other two images could be a worse case of the same or something else.
