Closed form solution of ‘Hard spin mean field theory’ is constructed
and applied to ‘Three dimensional stacked-triangular-system’. The phase
diagram of this system is examined. ‘Free energy’ is calculated to reveal the
thermodynamically stable state in the phase diagram. A new second order
phase transition line is found near the zero external magnetic field. Strong
evidence is found for the tricriticality point behaviour in the multicritical
region.