The dynamic stability properties of a pre-stressed incompressible elastic plate are studied in this paper with respect to perturbations in the form of one near-neutral mode and two non-neutral modes interacting resonantly. The pre-stresses are assumed to be an all-round pressure. With the aid of a novel derivation procedure, the evolution equations governing the scaled amplitudes of the th:'ee modes are found to _be g_iven by d2Al/dr 2 = -coAl -cllA112A1 -i3'IA2A3, dA2/dr -'~ 0'2~12~3 and dA3/dr = "y3AIA2, where a bar denotes complex conjugation, r is a slow time variable and co, cl, 71, "r2, 3' 3 are real constants. These equations are solved exactly for the special case when A2 and A3 have constant amplitudes but time-dependent phases. A series of new post-buckling states, which does not exist when the perturbation is monochromatic, are found. We show that two nonneutral modes can interact resonantly to produce a much larger near-neutral mode, and in particular, two O(¢) non-neutral modes may induce a much larger O(¢ 2/3) oscillation of static post-buckling state. In this sense, resonant-triad interaction is a powerful mechanism in producing high levels of strain and stress in a pre-stressed elastic plate.