We recently introduced a heuristic method for delimiting the stability regions of a three-mode class A laser with three freedom degrees. Here this method is extended to cover the stability conditions of a three-mode class B laser with five freedom degrees. The diagonal arrays of the stability coefficient determinant are taken equal to zero until the determinant vanishes. It then turns out that the diagonal roots correspond to the laser stability boundaries. They form the different kinds of bifurcations that segregate the above-threshold state, with three simultaneous oscillating modes, from the bistability and below-threshold states, with two and no oscillating modes, respectively. The fast optical switches and high optical memories have been designed by exploiting the bistability properties of bifurcations as reported by Perez et al (2007 Opt. Express 15 12941–8).