  We define the thin fundamental Gray 3-groupoid $S_3(M)$ of a smooth manifold
$M$ and define (by using differential geometric data) 3-dimensional holonomies,
to be smooth strict Gray 3-groupoid maps $S_3(M) \to C(H)$, where $H$ is a
2-crossed module of Lie groups and $C(H)$ is the Gray 3-groupoid naturally
constructed from $H$. As an application, we define Wilson 3-sphere observables.
