  Equations of motion for a classical 3d discrete model, whose auxialiary
system is a linear system, are investigated. The Lagrangian form of the
equations of motion is derived. The Lagrangian variables are a triplet of "tau
functions". The equations of motion for the Triplet of Tau functions are Three
Trilinear equations. Simple solitons for the trilinear equations are given.
Both the dispersion relation and the phase shift reflect the triplet structure
of equations.
