  We establish the well-posedness in Gevrey function space with optimal class
of regularity 2 for the three dimensional Prandtl system without any structural
assumption. The proof combines in a novel way a new cancellation in the system
with some of the old ideas to overcome the difficulty of the loss of
derivatives in the system.This shows that the three dimensional instabilities
in the system leading to ill-posedness are not worse than the two dimensional
ones.
