This paper addresses a 3D floorplanning methodology, which considers floorplanning and powerplanning at the same time for Multiple Supply Voltage (MSV) circuits. Physical design becomes more complex for MSV designs since modules with the same power domain have to be placed at close locations in 3D space to facilitate powerplanning and reduce IR-drop, which would deteriorate wirelength. By properly partitioning modules of the same power domain into several voltage islands and increasing overlap area of the voltage islands in contiguous dies, we can reduce routing resource usage without increasing wirelength significantly. Further, unlike previous works, our approach not only can handle a netlist with soft modules and hard modules but also can meet the fixed-outline constraint. The experimental results show that our methodology gets better results than other approach in designs with single voltage domain and is also promising for MSV designs.