 For 3D, the system is block tridiagonal and is best solved by one of the iterative solvers.In computation of two-fluid flows, in some cases the interface might be too complex to track while keeping the frequency of re-meshing at an acceptable level.Not being able to reduce the frequency of re-meshing in 3D might introduce overwhelming mesh generation and projection costs, making the computations with the interface-tracking technique no longer feasible.