  It is well known that quantum mechanics is incompatible with local realistic
theories. Svetlichny showed, through the development of a Bell-like inequality,
that quantum mechanics is also incompatible with a restricted class of nonlocal
realistic theories for three particles where any two-body nonlocal correlations
are allowed [Phys. Rev. D 35, 3066 (1987)]. In the present work, we
experimentally generate three-photon GHZ states to test Svetlichny's
inequality. Our states are fully characterized by quantum state tomography
using an overcomplete set of measurements and have a fidelity of (84+/-1)% with
the target state. We measure a convincing, 3.6 std., violation of Svetlichny's
inequality and rule out this class of restricted nonlocal realistic models.
