  The pressure, and the energy and entropy densities are determined for the
SU(3) gauge theory in $2 + 1$ dimensions from lattice Monte Carlo calculations
in the interval $0.6 \leq T/T_c \leq 15$. The finite temperature lattices
simulated have temporal extent $N_\tau = 2, 4, 6$ and 8, and spatial volumes
$N_S^2$ such that the aspect ratio is $N_S/N_\tau = 8$. To obtain the
thermodynamical quantities, we calculate the averages of the temporal
plaquettes $P_\tau$ and the spatial plaquettes $P_S$ on these lattices. We also
need the zero temperature averages of the plaquettes $P_0$, calculated on
symmetric lattices with $N_\tau = N_S$. We discuss in detail the finite size
($N_S$-dependent) effects. These disappear exponentially. For the zero
temperature lattices we find that the coefficient of $N_S$ in the exponent is
of the order of the glueball mass. On the finite temperature lattices it lies
between the two lowest screening masses. For the aspect ratio equal to eight,
the systematic errors coming from the finite size effects are much smaller than
our statistical errors. We argue that in the continuum limit, at high enough
temperature, the pressure can be parametrized by the very simple formula
$p=a-bT_c/T$ where $a$ and $b$ are two constants. Using the thermodynamical
identities for a large homogeneous system, this parametrization then determines
the other thermodynamical variables in the same temperature range.
