  We calculate the one-, two- and three-particle energy levels for different
lattice volumes in the complex $\varphi^4$ theory on the lattice. We argue that
the exponentially suppressed finite-volume corrections for the two- and
three-particle energy shifts can be reduced significantly by using the single
particle mass, which includes the finite-size effects. We show numerically
that, for a set of bare parameters, corresponding to the weak repulsive
interaction, one can reliably extract the two- and three-particle energy
shifts. From those, we extract the scattering length, the effective range and
the effective three-body coupling. We show that the parameters, extracted from
the two- and three-particle energy shifts, are consistent. Moreover, the
effective three-body coupling is significantly different from zero.
