Achieving high performance, multifunctionality, and a compact size simultaneously in a single T/R module presents a significant challenge. In this work, both the 'chiplet' technique and 3D integration technique are adopted to form a T/R microsystem using self-matching interconnectors. Such T/R microsystem consists of four GaAs chips and one CMOS chip. The designed microsystem with a small size is utilized for the application of tile-type mmWave active phased array. Its measured results demonstrate good performance in terms of high gain, large output power, 6-bit phase shift, and 6-bit attenuation, confirming the effectiveness of our adopted architecture and design method. As such, this work provides a valuable design guideline for the chiplet-based 3D integrated RF microsystem.