High-throughput three-dimensional (3D) focusing of cells is the key prerequisite for enabling accurate microfluidic cell detection and analysis. In this work, we develop a high-aspect-ratio asymmetric serpentine (HARAS) microchannel for single-line inertial focusing of particles and cells at the 3D center of the channel. The mechanism of 3D focusing is explored by numerical simulation, and the focusing performances of differently sized particles are characterized experimentally at different flow rates. The results demonstrate the outstanding 3D single-line focusing capability of our HARAS microchannel. In addition, the phenomena of size-independent and position-controllable focusing over wide flow rates are observed. Finally, the applicability of our HARAS microchannel for processing real biological cells is validated by the 3D single-line focusing of A549 cells and MCF-7 cells. Our work overcomes the issue of off-centered focusing of most previous works and provides new insights into the 3D focusing in inertial microfluidics. The proposed HARAS microchannel is extremely easy for mass production and may provide a stable, high-throughput, and position-controllable scheme for subsequent single-cell detection and analysis.