In this paper we
characterize those 3-manifolds M3 satisfying Z ⊕Z ⊕Z ⊆ π1(M). All such manifolds
M arise in one of the following ways: (I) M = M0#R, (II) M = M0#R∗, (III)
M = M0∪∂R∗. Here M0 is any 3-manifold in (I), (II) and any 3-manifold having P2
components in its boundary in (III). R is a flat space form and R∗ is obtained from R
and some involution i : R → R with fixed points, but only finitely many, as follows: if
C1,…,Cn are disjoint 3-cells around the fixed points then R∗ is the 3-manifold
obtained from (R−int(C1∪⋯∪Cn))∕i by identifying some pairs of projective planes
in the boundary.