A certain class of integrable hydrodynamic type systems with three
independent and N dependent variables is considered. We choose the existence of
a pseudopotential as a criterion of integrability. It turns out that the class
of integrable systems having pseudopotentials with movable singularities is
described by a functional equation, which can be solved explicitly. This allows
us to construct interesting examples of integrable hydrodynamic systems for
arbitrary N.Comment: 12 pages, late