  For zero-range interaction providing a given mass M_2 of the two-body bound
state, the mass M_3 of the relativistic three-body bound state is calculated.
We have found that the three-body system exists only when M_2 is greater than a
critical value M_c (approx. 1.43m for bosons and approx. 1.35m for fermions, m
is the constituent mass). For M_2=M_c the mass M_3 turns into zero and for
M_2<M_c there is no solution with real value of M_3.
