This paper treats the placement of stacked 3D modules in a stacked-3D LSI, and proposes a coding scheme named Triplet-Sequence. A code word of Triplet-Sequence consists of two sequences of module names which work similarly but a bit differently with Sequence Pair and one other sequence of integral numbers. Another important contribution is the decoding algorithm for flexible Stacked 3-D modules. The shape of each 3D module is not limited to a cuboid, but a collection of vertically-stacked rectangles which may have individual sizes and relative positions in the (x,y)-coordinate (positional offsets). A flexible module is defined as a stacked 3-D module with variable positional offsets. The application of Simulated Annealing Search to the solution space defined with Triplet-Sequence is demonstrated to show the validity of flexible modules and Triplet-Sequence as one of the possible design models for Stacked 3-D layout.