The word "they" here refers to the people of Judah. These three phrases mean about the same thing and are repeated for emphasis. (See: [[rc://en/ta/man/jit/figs-parallelism]])
This can be expressed in active form. Alternate translation: "No one will gather their bones or bury them again" (See: [[rc://en/ta/man/jit/figs-activepassive]])
The bones are compared to dung on the ground both to emphasize that they will be disgusting and because they will not be buried. (See: [[rc://en/ta/man/jit/figs-simile]])