  The $(2+1)$-dimensional $q=3$ Potts model was simulated with the exact
diagonalization method. In the ordered phase, the elementary excitations
(magnons) are attractive, forming a series of bound states in the low-energy
spectrum. We investigate the low-lying spectrum through a dynamical
susceptibility, which is readily tractable with the exact diagonalization
method via the continued-fraction expansion. As a result, we estimate the
series of (scaled) mass gaps, $m_{2,3,4}/m_1$ ($m_1$: single-magnon mass), in
proximity to the transition point.
