  We study generation and dynamics of an exchange spin qubit encoded in three
coherently coupled quantum dots with three electrons. For two geometries of the
system a linear and a triangular one, the creation and coherent control of the
qubit states are performed by the Landau--Zener transitions. In the triangular
case both the qubit states are equivalent and can be easily generated for
particular symmetries of the system. If one of the dots is smaller than the
others one can observe Rabi oscillations, that can be used for coherent
manipulation of the qubit states. The linear system is easier to fabricate;
however, then the qubit states are not equivalent, making qubit operations more
difficult to control.
