  In this note we derive a new Minkowski-type inequality for closed convex
surfaces in the hyperbolic 3-space. The inequality is obtained by explicitly
computing the area of the family of surfaces obtained from the normal flow and
then applying the isoperimetric inequality. Using the same method, we also we
give elementary proofs of the classical Minkowski inequalities for closed
convex surfaces in the Euclidean 3-space and in the 3-sphere.
