  In a recent paper, Liu, Zhu and Wu (2015, {\it J. Fluid Mech.} {\bf 784}:
304) present a force theory for a body in a two-dimensional, viscous,
compressible and steady flow. In this companion paper we do the same for
three-dimensional flow. Using the fundamental solution of the linearized
Navier-Stokes equations, we improve the force formula for incompressible flow
originally derived by Goldstein in 1931 and summarized by Milne-Thomson in
1968, both being far from complete, to its perfect final form, which is further
proved to be universally true from subsonic to supersonic flows. We call this
result the \textit{unified force theorem}, which states that the forces are
always determined by the vector circulation $\pGamma_\phi$ of longitudinal
velocity and the scalar inflow $Q_\psi$ of transverse velocity. Since this
theorem is not directly observable either experimentally or computationally, a
testable version is also derived, which, however, holds only in the linear far
field. We name this version the \textit{testable unified force formula}. After
that, a general principle to increase the lift-drag ratio is proposed.
