  We consider disorder-order phase transitions in the three-dimensional version
of the scalar noise model (SNM) of flocking. Our results are analogous to those
found for the two-dimensional case. For small velocity (v <= 0.1) a continuous,
second-order phase transition is observable, with the diffusion of nearby
particles being isotropic. By increasing the particle velocities the phase
transition changes to first order, and the diffusion becomes anisotropic. The
first-order transition in the latter case is probably caused by the interplay
between anisotropic diffusion and periodic boundary conditions, leading to a
boundary condition dependent symmetry breaking of the solutions.
