  The volume-dependence of a shallow three-particle bound state in the cubic
box with a size $L$ is studied. It is shown that, in the unitary limit, the
energy-level shift from the infinite-volume position is given by $\Delta E=c
(\kappa^2/m)\,(\kappa L)^{-3/2}|A|^2 \exp(-2\kappa L/\sqrt{3})$, where $\kappa$
is the bound-state momentum and $|A|^2$ denotes the three-body analog of the
asymptotic normalization constant, which encodes the information about the
short-range interactions in the three-body system.
