  We consider the transition of electron between two quantum dots in which the
discrete levels are swept past each other with a constant velocity. If a direct
tunneling between the dot levels was allowed, an electron will be transferred
between the dots when the levels cross. This transfer is described in terms of
the conventional Landau-Zener theory. We assume that direct tunneling between
the dots is forbidden. Rather, the transfer is due to the resonant tunneling
via a discrete impurity level separating the dots. Then the description of the
electron transfer reduces to a threestate (two dots plus impurity) Landau-Zener
transition. Transition probability depends on the relative positions of the
resonant level and the energy at which the levels cross. It also depends on the
left-right asymmetry of tunneling between the impurity and the left(right)
dots. We calculate the transition probability in different limits of the
horizontal (in space) and vertical (in energy) impurity level positions.
