  It is well known that a three dimensional (closed, connected and compact)
manifold is obtained by identifying boundary faces from a polyhedron P. The
study of (\partial P)/~, the boundary \partial P with the polygonal faces
identified in pairs leads us to the following conclusion: either a three
dimensional manifold is homeomorphic to a sphere or to a polyhedron P with its
boundary faces identified in pairs so that (\partial P)/~ is a finite number of
internally flat complexes attached to each other along the edges of a finite
graph that contains at least one closed circuit. Each of those internally flat
complexes is obtained from a polygon where each side may be identified with
more than one different sides. Moreover, Euler characteristic of (\partial P)/~
is equal to one and the fundamental group of (\partial P)/~ is not trivial.
