  In this paper, we consider a class of Morse-Smale diffeomorphisms defined on
a closed 3-manifold (non-necessarily orientable) under the assumption that all
their saddle points have the same dimension of the unstable manifolds. The
simplest example of such diffeomorphisms is the well-known ``source-sink'' or
``north pole - south pole'' diffeomorphism, whose non-wandering set consists of
exactly one source and one sink. Such systems, as Reeb showed back in 1946, can
be realized only on the sphere. We generalize his result, namely, we show that
diffeomorphisms from the considered class also can be defined only on the
3-sphere.
