The diffraction trace formula derived previously and the
spectral determinant are tested on the open three-disk
scattering system. The system contains a generic and
exponentially growing number of diffraction periodic orbits. In
spite of this it is shown that even the scattering resonances
with large imaginary part can be reproduced semiclassically. The
nontrivial interplay of the diffraction periodic orbits with the
usual geometrical orbits produces the fine structure of the
complicated spectrum of scattering resonances, which are beyond
the resolution of the conventional periodic orbit theory.
