The present work proposes a micromixer consisting of circular mixing modules and three-dimensional subchannels, which uses a split and recombination process and unbalanced collisions as the main mixing mechanisms. Three micromixer configurations with different locations of the forward- and backward-facing steps were tested at several Reynolds numbers between 0.1 and 120. The steady incompressible momentum and continuity equations in conjunction with the advection-diffusion equation were solved for the analysis of mixing and fluid flow. The micro mixer configuration with three-dimensional step structures in both subchannels showed the most excellent mixing capability. In comparison with a previous micromixer of the same type, the suggested micromixer demonstrates far better efficiency at medium Reynolds numbers. The number of mixing modules and geometric parameters were also varied to estimate its impact on the mixing capability. The suggested micromixer exhibited a mixing index higher than 0.86 for Re >= 20 and 0.95 for Re >= 50.