  We consider the stability of three Coulomb charges $\{+1, -1, -1 \}$ with
finite masses in the framework of nonrelativistic quantum mechanics. A simple
physical condition on masses is derived to guarantee the absence of bound
states below the dissociation thresholds. In particular this proves that
certain negative muonic ions are unstable, thus extending the old result of
Thirring \cite{thirring} to the actual values of all masses. The proof is done
by reducing the initial problem to the question of binding of one particle in
some effective potential.
