  We analyze a mesoscopic model of a shear stress material with a three
dimensional slab geometry, under an external quasistatic deformation of a
simple shear type. Relaxation is introduced in the model as a mechanism by
which an unperturbed system achieves progressively mechanically more stable
configurations. Although in all cases deformation occurs via localized plastic
events (avalanches) we find qualitatively different behavior depending on the
degree of relaxation in the model. For no or low relaxation yielding is
homogeneous in the sample, and even the largest avalanches become negligible in
size compared with the system size (measured as the thickness of the slab
$L_z$) when this is increased. On the contrary, for high relaxation the
deformation localizes in an almost two dimensional region where all avalanches
occur. Scaling analysis of the numerical results indicates that in this case
the linear size of the largest avalanches is comparable with $L_z$ even when
this becomes very large. We correlate the two scenarios with a qualitative
difference in the flow curve of the system in the two cases, which is
monotonous in the first case, or of the velocity weakening type in the second
case.
