The sequence-pair method is a sufficient and finite (P-admissible) coding method for solution space representation of 2D rectangle packing problem, and is proved from a graphical demonstration called Gridding Procedure. Inspired by the 2D sequence-pair method, there should also be sequence-triplet method for 3D rectangle packing problem. But in 3D space, the gridding procedure is difficult to be realized. In this paper, a way is introduced to achieve the sequence-triplet method for 3D rectangle packing problem by analysis and deduction of matrix representation of sequence and partial sequence. This sequence-triplet coding method is P-admissible.