  We consider a pairwise interacting quantum 3-body system in 3-dimensional
space with finite masses and the interaction term $V_{12} + \lambda(V_{13} +
V_{23})$, where all pair potentials are assumed to be nonpositive. The pair
interaction of the particles $\{1,2\}$ is tuned to make them have a zero energy
resonance and no negative energy bound states. The coupling constant $\lambda
>0$ is allowed to take the values for which the particle pairs $\{1,3\}$ and
$\{2,3\}$ have no bound states with negative energy. Let $\lambda_{cr}$ denote
the critical value of the coupling constant such that $E(\lambda) \to -0$ for
$\lambda \to \lambda_{cr}$, where $E(\lambda)$ is the ground state energy of
the 3-body system. We prove the theorem, which states that near $\lambda_{cr}$
one has $E(\lambda) = C (\lambda-\lambda_{cr})[\ln
(\lambda-\lambda_{cr})]^{-1}+$h.t., where $C$ is a constant and h.t. stands for
"higher terms". This behavior of the ground state energy is universal (up to
the value of the constant $C$), meaning that it is independent of the form of
pair interactions.
