Unfitted finite element methods (FEMs) are powerful tools for addressing challenges related to deformable and moving surfaces, particularly when large deformations are involved. These methods are especially relevant for studying cell migration in 3D environments. In this work, the focus is on freely swimming cells in a 3D viscous fluid, a scenario encountered by immune cells like leukocytes in the bloodstream or cancer cells navigating complex 3D environments. Self-sustained cell motility arises from the contractility and deformability of the actomyosin cortex, a dynamic system that enables cells to generate and adapt their movement. The application of Unfitted FEMs to cell migration is natu- ral, as these methods excel at handling complex and dynamic geometries. In this talk, we will introduce a novel and robust unfitted FEM designed to address coupled surface-bulk viscous flows and its application to 3D cell migration. The finite element discretisation combines the Aggregated Finite Element Method for the bulk phases with the Trace Finite Element Method for the surface problem. Numerical results are presented, demonstrating how migration is influenced by the viscosity of cortex and the activity of the cortical layer, while viscosities of the different fluid phases and the boundary conditions don’t affect the swimming velocity. These findings provide valuable insights into the mechanics of cell migration and its dependence on environmental factors.