Masonry arch bridges, widely found across Europe, represent a critical yet vulnerable component of transportation infrastructure. Their complex geometries and aged materials pose significant challenges for effective Structural Health Monitoring (SHM), particularly in terms of vibration-based dynamic identification. This study presents an optimized sensor placement strategy for a reduced-scale masonry arch bridge specimen constructed at the Universitat Politècnica de Catalunya, within the framework of the PONT3 Project. A detailed finite element (FE) model was developed to perform modal analysis and identify significant mode shapes, serving as the foundation for evaluating sensor layouts. The Effective Independence (EfI) algorithm and its triaxial extension (EfI3) were used to determine optimal sensor configurations for both uniaxial and triaxial accelerometers. Results indicate that while uniaxial configurations can capture natural frequencies, they fall short in providing sufficient spatial resolution for accurate mode shape reconstruction. In contrast, the EfI3-based triaxial setup achieves more reliable and interpretable modal data with fewer sensors, a crucial benefit for practical deployment. These findings not only enhance the accuracy of FE model updating but also contribute to the broader goal of developing cost-effective SHM frameworks tailored for masonry arch bridges.