In this paper, the efficiency of the diakoptic concept applied to three-dimensional filter modelling is shown. The technique is implemented in the Frequency-Domain Transmssion Line Matrix (FD-TLM) method using the Hybrid Symmetrical Condensed Node (HSCN) and a variable mesh-size. Thus, the structure is decomposed into several sub-domains and smaller nodes are applied in the subdomain to be computed. An important reduction in computational time and memory resources is achieved with a good accuracy.