Six of these lines are the connective 을뿐더러, meaning not only that but also: there are only three workers, but they are good at the job and pleasant with it; bones have no nutrients and are far too hard as well. The other two are a different thing entirely, the particle 더러, an old-fashioned way of marking the person told or asked, as in the lines meaning she said to me and this means I am the one who is supposed to serve them.
Register: the 뿐더러 use is neutral to formal; the particle is old-fashioned