  We present three-dimensional hybrid simulations of collisionless shocks that
propagate parallel to the background magnetic field to study the acceleration
of protons that forms a high-energy tail on the distribution. We focus on the
initial acceleration of thermal protons and compare it with results from
one-dimensional simulations. We find that for both one- and three-dimensional
simulations, particles that end up in the high-energy tail of the distribution
later in the simulation gained their initial energy right at the shock. This
confirms previous results but is the first to demonstrate this using fully
three-dimensional fields. The result is not consistent with the "thermal
leakage" model. We also show that the gyrocenters of protons in the
three-dimensional simulation can drift away from the magnetic field lines on
which they started due to the removal of ignorable coordinates that exist in
one- and two-dimensional simulations. Our study clarifies the injection problem
for diffusive shock acceleration.
