  We extend our previous work on the 2D Dimits transition in ion-scale
turbulence (Ivanov et al. 2020) to include variations along the magnetic field.
We consider a three-field fluid model for the perturbations of electrostatic
potential, ion temperature, and ion parallel flow in a
constant-magnetic-curvature geometry without magnetic shear. It is derived in
the cold-ion, long-wavelength asymptotic limit of the gyrokinetic theory. Just
as in the 2D model, a low-transport (Dimits) regime exists and is found to be
dominated by a quasi-static staircase-like arrangement of strong zonal flows
and zonal temperature. This zonal staircase is formed and maintained by a
negative turbulent viscosity for the zonal flows. Unlike the 2D model, the 3D
one does not suffer from an unphysical blow up beyond the Dimits threshold
where the staircase becomes nonlinearly unstable. Instead, a well-defined
finite-amplitude saturated state is established. This qualitative difference
between 2D and 3D is due to the appearance of small-scale `parasitic' modes
that exist only if we allow perturbations to vary along the magnetic field
lines. These modes extract energy from the large-scale perturbations and
provide an effective enhancement of large-scale thermal diffusion, thus aiding
the energy transfer from large injection scales to small dissipative ones. We
show that in our model, the parasitic modes always favour a
zonal-flow-dominated state. In fact, a Dimits state with a zonal staircase is
achieved regardless of the strength of the linear drive provided the system is
sufficiently extended along the magnetic field and sufficient parallel
resolution is provided.
