  We use imaging from the first three years of the Dark Energy Survey to
characterize the dynamical state of 288 galaxy clusters at $0.1 \lesssim z
\lesssim 0.9$ detected in the South Pole Telescope (SPT) Sunyaev-Zeldovich (SZ)
effect survey (SPT-SZ). We examine spatial offsets between the position of the
brightest cluster galaxy (BCG) and the center of the gas distribution as traced
by the SPT-SZ centroid and by the X-ray centroid/peak position from Chandra and
XMM data. We show that the radial distribution of offsets provides no evidence
that SPT SZ-selected cluster samples include a higher fraction of mergers than
X-ray-selected cluster samples. We use the offsets to classify the dynamical
state of the clusters, selecting the 43 most disturbed clusters, with half of
those at $z \gtrsim 0.5$, a region seldom explored previously. We find that
Schechter function fits to the galaxy population in disturbed clusters and
relaxed clusters differ at $z>0.55$ but not at lower redshifts. Disturbed
clusters at $z>0.55$ have steeper faint-end slopes and brighter characteristic
magnitudes. Within the same redshift range, we find that the BCGs in relaxed
clusters tend to be brighter than the BCGs in disturbed samples, while in
agreement in the lower redshift bin. Possible explanations includes a higher
merger rate, and a more efficient dynamical friction at high redshift. The
red-sequence population is less affected by the cluster dynamical state than
the general galaxy population.
