  We recently derived a quantization condition for the energy of three
relativistic particles in a cubic box. Here we use this condition to study the
energy level closest to the three-particle threshold when the total
three-momentum vanishes. We expand this energy in powers of $1/L$, where $L$ is
the linear extent of the finite volume. The expansion begins at ${\cal
O}(1/L^3)$, and we determine the coefficients of the terms through ${\cal
O}(1/L^6)$. As is also the case for the two-particle threshold energy, the
$1/L^3$, $1/L^4$ and $1/L^5$ coefficients depend only on the two-particle
scattering length $a$. These can be compared to previous results obtained using
nonrelativistic quantum mechanics and we find complete agreement. The $1/L^6$
coefficients depend additionally on the two-particle effective range $r$ (just
as in the two-particle case) and on a suitably defined threshold three-particle
scattering amplitude (a new feature for three particles). A second new feature
in the three-particle case is that logarithmic dependence on $L$ appears at
$\mathcal O(1/L^6)$. Relativistic effects enter at this order, and the only
comparison possible with the nonrelativistic result is for the coefficient of
the logarithm, where we again find agreement. For a more thorough check of the
$1/L^6$ result, and thus of the quantization condition, we also compare to a
perturbative calculation of the threshold energy in relativistic $\lambda
\phi^4$ theory, which we have recently presented. Here all terms can be
compared and we find full agreement.
