  The Klein-Gordon system describing three scalar particles without interaction
is cast into a new form, by transformation of the momenta. Two redundant
degrees of freedom are eliminated; we are left with a covariant equation for a
reduced wave function with three-dimensional arguments. This new formulation of
the mass-shell constraints is equivalent to the original KG system in a sector
characterized by positivity of the energies and, if the mass differences are
not too large, by a moderately relativistic regime. Then, mutual interaction is
introduced and produces a tractable model.
