Three-dimensional particle focusing is a prerequisite for a wide range of lab-on-a-chip applications such as cell counting and sorting. We have demonstrated that when side-wells are inserted in a rectangular channel, the particles form an almost single stream (> 99 %) within +/- 0.9x particle diameter from the channel centerline under viscoelastic flow. Recently, viscoelasticity-based particle focusing technique has attracted much attention since it operates by flowing particles in extremely simple channels. However, the particles move along multiple equilibrium positions in rectangular channels under elasticity-dominant flow, and this is not favorable for practical applications. We show that the hoop stress along curved streamlines in side-wells can be engineered to reduce the multiple particle lanes to a single stream. Further, we achieve highly efficient focusing under inertialess viscoelastic flow with high throughput (> 2000/s). We expect that our novel method will find applications in accurate cell counting, sorting, and deformability measurement.