We propose a scheme for efficient cavity-enhanced nonlinear THz generation via difference- frequency generation (DFG) processes using a triply resonant system based on photonic crystal cavities. We show that high nonlinear overlap can be achieved by coupling a THz cavity to a doubly-resonant, dual-polarization near-infrared (e.g. telecom band) photonic-crystal nanobeam cavity, allowing the mixing of three mutually orthogonal fundamental cavity modes through a second-order nonlinearity. We demonstrate through coupled-mode theory that complete depletion of the pump frequency (quantum-limited conversion) is possible in an experimentally feasible geometry, with the operating output power at the point of optimal total conversion e±ciency tunable by varying the mode quality (Q) factors.