  We present a 3D hydrodynamics study of gravitational instabilities (GIs) in a
0.14 M$_{\odot}$ circumbinary protoplanetary disc orbiting a 1 M$_{\odot}$ star
and a 0.02 M$_{\odot}$ brown dwarf companion. We examine the thermodynamical
state of the disc and determine the strengths of GI-induced density waves,
nonaxisymmetric density structures, mass inflow and outflow, and gravitational
torques. Results are compared with a parallel simulation of a protoplanetary
disc without the brown dwarf binary companion. Simulations are performed using
CHYMERA, a radiative 3D hydrodynamics code. The onset of GIs in the
circumbinary disc is much more violent due to the stimulation of a strong
one-armed density wave by the brown dwarf. Despite this early difference,
detailed analyses show that both discs relax to a very similar quasi-steady
phase by 2,500 years after the beginning of the simulations. Similarities
include the thermodynamics of the quasi-steady phase, the final surface density
distribution, radial mass influx, and nonaxisymmetric power and torques for
spiral arm multiplicities of two or more. Effects of binarity in the disc are
evident in gravitational torque profiles, temperature profiles in the inner
discs, and radial mass transport. After 3,800 years, the semimajor axis of the
binary decreases by about one percentage and the eccentricity roughly doubles.
The mass transport in the outer circumbinary disc associated with the one-armed
wave may influence planet formation.
