  We propose that Hofstadter's butterfly accompanied by quantum Hall effect
that is similar to those predicted to occur in 3D tight-binding systems by
Koshino {\it et al.} [Phys. Rev. Lett. {\bf 86}, 1062 (2001)] can be realized
in an entirely different system -- 3D metals applied with weak external
periodic modulations (e.g., acoustic waves). Namely, an effect of two periodic
potentials interferes with Landau's quantization due to an applied magnetic
field $\Vec{B}$, resulting generally in fractal energy gaps as a function of
the tilting angle of $\Vec{B}$, for which the accompanying quantized Hall
tensors are computed. The phenomenon arises from the fact that, while the
present system has a different physical origin for the butterfly from the 3D
tight-binding systems, the mathematical forms are remarkably equivalent.
