The diffraction trace formula ({\em Phys. Rev. Lett.} {\bf 73}, 2304 (1994))
and 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 semiclassicaly. The
non-trivial 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.Comment: Latex article + 3 ps figure
