We demonstrated a high-quality single-photon emitter based on excitation energy transfer between two different-sized CuCl quantum dots (QDs). It is operated under triple blockade mechanisms. The mechanisms are tuning the incident light to the smaller QD and using an electric dipole forbidden excitation energy level in the larger QD, using the optical near-field interaction to transfer energy from the smaller QD to the electric dipole forbidden level of the larger QD, and using a single exciton emission level in the larger QD. These mechanisms are supported by the large binding energy of the exciton molecule. A 99.3% plausibility of single-photon emission was confirmed with 99.98% accuracy based on a photon correlation experiment with 80-MHz repetition frequency using an optical fiber probe.