You can use the 3D Layer Configurator to enter the third dimension. In the main menu, please click on [b]Edit[/b] and then on [b]3D Layer Configuration...[/b]. In the upper left corner, there are buttons to insert slice(s) above and below with a user defined dictance.
Please note that FEFLOW is not designed to simulate different miscible / immiscible fluids. FEFLOW can be applied as long as the system can be represented with a single fluid and one or more chemical species whose drivers are advection, diffusion, dispersion and optionally also convection.