Parametric resonance has been observed, both numerically and experimentally, in various studies of wave energy converters (WECs). Large heave motions induce a periodic variation in the metacentric height of a WEC body and, consequently, cause a harmonic variation in pitch/roll restoring coefficients, which can parametrically excite the pitch/roll modes. Current studies attempt to determine the onset conditions of parametric resonance, by detecting the boundaries between stable and unstable regions in the parameter space. In the literature, some studies aim to make use of parametric resonance for improving power capture. In contrast, some studies try to suppress the effect of parametric resonance, as it can reduce power capture efficiency in the primary degree of freedom. However, how energy transfers from one mode to another is not fully understood. This study aims to analyse energy transfer between heave and pitch/roll modes when parametric resonance occurs. A generic cylindrical point absorber is studied as a WEC floater to consider non-linear wave-structure interaction, including non-linear Froude-Krylov and viscous forces. A heave-pitch-roll three-degree-of-freedom model is derived for numerical study of the energy transfer between different operational modes.