A cavity-aperture has a problem of low transmission efficiency due to its nano-sized aperture despite its potential for plasmonic color filters. In this study, a triple-slit aperture is proposed as the nanoaperture in the center of the cavity-aperture to improve the transmittance. It provides one centered nanoslit and two symmetric wedge structures to each of three cavities corresponding to incident polarization, and induces the strong confinement and transmission of electric fields due to plasmonic resonances at the two types of nanostructures. The transmittance of the triple-slit aperture is theoretically five times and experimentally two times higher than that of a circular aperture. Furthermore, expansive studies on polarization-insensitive nanoapertures with six-fold rotational symmetry will contribute to the development of plasmonic color filters and imaging devices.