Abstract
The motion of three free particles is studied in their center-of-mass frame. In the six-dimensional space of relative momenta the invariant mass is used as a radial variable. Curves perpendicular to surfaces of constant mass ("mass-shells") are labelled by their direction at threshold mass. The relativistic distortion from the nonrelativistic case when the mass shells are ellipsoids is studied. This means a mapping in the radial variable and two of the angular variables (Dalitz plot variables). Since an analytic treatment of the problem seems hardly feasible a numerical study has been made.