  This is the last part of a series of three strongly related papers in which
three equivalent structures are studied:
  - internal categories in categories of monoids; defined in terms of pullbacks
relative to a chosen class of spans
  - crossed modules of monoids relative to this class of spans
  - simplicial monoids of so-called Moore length 1 relative to this class of
spans.
  The most important examples of monoids that are covered are small categories
(treated as monoids in categories of spans) and bimonoids in symmetric monoidal
categories (regarded as monoids in categories of comonoids). In this third part
relative simplicial monoids are analyzed. Their Moore length is introduced and
the equivalence is proven between relative simplicial monoids of Moore length
1, and relative categories of monoids in Part I. This equivalence is obtained
in one direction by truncating a simplicial monoid at the first two degrees;
and in the other direction by taking the simplicial nerve of a relative
category.
