The admittance expansion to higher order, both in external flux and in junction phase, gives a more complete model for the three-wave degenerate case. The even harmonics in the flux expansion serve to only modify the net inductance value. The odd harmonics modify the potential gain and phase-sensitivity. This allows for estimation of the saturation effects. As it is in the theory of mixers, we see in the ac-flux expansion that the third-order term compresses the gain-providing first-order term.