In the present study, an accurate general-purpose three-dimensional computer program has been developed for hydroelastic response of Very Large Floating Structures (VLFSs) due to regular waves in water area of complicated environment close to real situation. It has pursued a well-balanced analysis of fluid-structure interactions with detailed modeling of respective media. Thus, NASTRAN has been adopted for the structural modeling and analysis for complicated VLFS structures and a hybrid finite/infinite element method of domain decomposition type has been newly developed for modeling and simulating the surrounding flow field of water-waves and then a modal interaction approach has been selected to make possible separate FE discretizations for fluid and structures. The present paper discusses the structural modeling, the fluid-structure interaction strategy and a large scale modal hydroelastic response scheme examined, following the first and the second reports on a large scale computation of water waves. Special attention has been paid to data transfer between NASTRAN and the corresponding hydroelastic interaction program and modal interaction schemes. Calculated hydroelastic responses for a rectangular plate 4, 800 m long have agreed well with the existing results. In addition, pilot calculations for a T-shaped airport and a highly asymmetric VLFS under construction have been successfully simulated. These have proved the applicability of this scheme to a class of VLFSs under consideration, largest in size and most complicated in shape.