This communication develops a triple-band shared- aperture antenna operating at X/K/Ka-band with high figures of merit such as high aperture reuse efficiency, simple configuration to achieve high gain. The proposed antenna is implemented by integrating a Fabry-Perot (FP) cavity antenna operating at X-band into a dual-band reflectarray antenna enabled by frequency selective surfaces (FSSs). The unit cells (UCs) to form the phase-shifting surfaces for the dual-band reflectarray antenna can offer partial reflectance/transmission abilities at X-band simultaneously, thereby enabling the implementation of a FP cavity antenna. The frequency responses of the proposed antenna are highly independent at X/K/Ka-band, which facilitates the design and optimization. For demonstration, a prototype has been described, fabricated, and measured. The simulated results present 3-dB gain bandwidths of 11.2 % (8.4 to 9.4 GHz), 10.0% (24 to 26.6 GHz), and 13.3% (35 to 40 GHz), which are all experimentally verified. The measured results also present a peak gain of 16.8 dBi at 9.0 GHz, 23.8 dBi at 26 GHz, and 26.7 dBi at 38 GHz.