  Darboux transformations are viewed as morphisms in a Darboux category.
Darboux transformations of type I which we defined previously, make an
important subgroupoid consists of Darboux transformations of type I. We
describe the orbits of this subgroupoid for hyperbolic operators of order
three.
  We consider the algebras of differential invariants for our operators. In
particular, we show that the Darboux transformations of this class can be
lifted to transformations of differential invariants (which we calculate
explicitly).
