The particle focus in the channel flow refers to a randomly initialized particle finally running at an equilibrium position at the channel cross section. The binding focus is a particle focus phenomenon that comprises two adjacent particles (including one rigid and the other soft), where particles can form and share a new equilibrium position. In this study, the result suggests that migrating the rigid particle laterally can lead to a passive migration of the soft. The above phenomenon is termed external force attached binding focus (EFABF). The EFABF is modeled to be three-dimensional using the immersed boundary-lattice Boltzmann method. The inertial focus of a single particle and the binding focus of two particles are numerically confirmed to validate the model. The migrating conditions of the soft particle are mainly discussed to further investigate the conditions of EFABF. Two patterns to migrate the soft particle are observed, including rigid particle ahead and soft particle ahead. The Reynolds number of 10 is proposed, which can apply to EFABF to obtain a significant migration scope. Moreover, the mechanism of EFABF is further analyzed to gain more insight into EFABF. Finally, as its application, a label-free single-cell separation architecture is designed by replacing the soft particle with a spherical circulating tumor cell and magnetically manipulating the rigid particle. The numerical results suggest that the soft particle (cell) can be well driven to stride over streamlines and form a new equilibrium position by migrating the rigid particle, thus making the separation pathway well-controlled.