Covariant Model for Relativistic Three-Body Systems
  The system is described by three mass-shell constraints. After a nonlinear transformation of the momenta, the analytic form taken by admissible interactions (allowing compatibility) is characterized in terms of the new variables. These variables mix two-body clusters, which results in automatically incorporating three-body forces. Two superfluous degrees of freedom are eliminated, which yields a reduced equation for a wave function depending on three-dimensional arguments. When at least two masses are equal, this picture has a reasonable nonrelativistic limit. At first post-Galilean order and provided the interaction is not too much energy-dependent, the relativistic correction is tractable like a conventional perturbation problem. A covariant version of harmonic oscillator is given as a toy model. 
Philippe Droz-Vincent