  Interfacial phenomena associated with fluid adsorption in two dimensional
systems has recently been shown to exhibit hidden symmetries, or covariances,
which precisely relate local adsorption properties in different confining
geometries. We show that covariance also occurs in three dimensional systems
and is likely to be verifiable experimentally and in Ising model simulations
studies. Specifically, we study complete wetting in wedge (W) and cone (C)
geometries as bulk coexistence is approached and show that the equilibrium
mid-point heights satisfy $l_c (h,\alpha)=l_w(\frac{h}{2} ,\alpha),$ where $h$
measures the partial pressure and $\alpha$ is the tilt angle. This covariance
is valid for both short-ranged and long-ranged intermolecular forces and
identifies both leading and next-to-leading order critical exponents and
amplitudes in the confining geometries. Connection with capillary
condensation-like phenomena is also made.
