  We extended existing theory of the two-pulse electric-dipole echo in glasses
in a magnetic field to the three-pulse echo. As is well known, at low
temperatures two-level systems (TLS's) are responsible for the echo phenomenon
in glasses. Using a diagram technique in the framework of perturbation theory
we derived a simple formula for the three-pulse echo amplitude. As in the case
of two-pulse echo the magnetic field dependence of the tree-pulse echo
amplitude in glasses is related to quadrupole electric moments of TLS's
non-spherical nuclei and/or dipole-dipole interaction of their nuclear spins.
These two mechanisms are responsible for the fine level splitting of TLS. As a
result TLS transforms to multi-level system with the fine level splitting
depending on a magnetic field. Due to existence in the theory the additional
parameter T - the time interval between the second and the third pulses we have
more reach spectrum of echo oscillations in a magnetic field in comparison with
the case of the two-pulse echo.
