The sparse aperture system gives a novel solution for the design of large telescopes with high angular resolution. Such systems utilize several smaller subapertures to simulate the full aperture but are more promising by virtue of their lighter weight and lower cost. Multiple-mirror telescopes (MMTs) and multiple-telescope telescopes (MTTs) are two types of sparse aperture systems. Here, the entrance pupil characteristics are derived for the Golay 3 MMT by studying the relationships between the submirrors on the primary mirror and the shapes of the subapertures on the entrance pupil with different relative apertures and conic constants of the primary mirror of the Golay 3 MMT. Overlay and efficiency factors are proposed in order to obtain the optimal fill factor on the entrance pupil. Expressions for the point spread function (PSF) and the modulation transfer function (MTF) of the Golay 3 MMT are deduced, and the characteristics of the PSF and MTF are explored based on the derived equations and Matlab simulation. Designs for two Cassegrain telescopes with the Golay3 configuration have been developed. Based on the relationships between the fill factor and the overlay factor, optimal parameters for the submirrors on the primary mirror can be selected. The PSF and MTF for both Cassegrain MMT designs have been obtained by Zemax simulation to prove that our theoretical results are consistent with those of practical simulation.