Arch dams are often preferred in narrow, high valleys due to their distinctively high and curved geometries, making them particularly effective for water retention. Consequently, assessing the seismic performance of these critical structures is essential. The presence of gallery openings within arch dams further heightens research interest, especially in regions of high seismic activity. This study presents a detailed three-dimensional (3D) numerical failure analysis of the Ermenek Dam, one of Turkey's largest arch dams. Using the finite- difference method, a 3D model is constructed, incorporating four distinct D- shaped gallery openings within the dam body. These openings are integrated into the model using specialized FLAC3D fish functions. To enhance simulation accuracy, the foundation is proportionally extended along the dam's base and sides. The Burgers creep material model is applied to assess both creep behavior and seismic response within the concrete body and foundation. Non- reflecting boundary conditions are imposed on the model boundaries to create realistic constraints. Numerical analyses are conducted on various gallery dimensions, evaluating 28 distinct geometries under ten different strong ground motions. The seismic analysis results indicate that damage may occur within the dam body when gallery height exceeds twice its width. Therefore, careful consideration of the geometrical dimensions of gallery openings is highly recommended during the modeling and analysis of arch dams.