  We study nonlinear Schr\"odinger equations, posed on a three dimensional
Riemannian manifold $M$. We prove global existence of strong $H^1$ solutions on
$M=S^3$ and $M=S^2\times S^1$ as far as the nonlinearity is defocusing and
sub-quintic and thus we extend the results of Ginibre-Velo and Bourgain who
treated the cases of the Euclidean space $\R^3$ and the flat torus $\T^3$
respectively. The main ingredient in our argument is a new set of multilinear
estimates for spherical harmonics.
