While numerous experimental studies have demonstrated the feasibility of reconfigurable intelligent surface (RIS) technology, most have primarily focused on extending coverage. In contrast, this paper presents an experimental evaluation of multiple active RISs deployed in a 5G multiple-input multiple-output (MIMO) commercial network, emphasizing enhancements in channel rank and throughput. We propose a low-complexity, codebook-based beamforming algorithm specifically tailored for multi-RIS configurations, which diversifies directional channels and reduces reliance on explicit channel state information. Field tests using a commercial base station and user equipment reveal that the multi-RIS system can improve channel rank and throughput by up to 14% compared to single-RIS deployments, while maintaining low computational complexity. These findings underscore the practical benefits of active multi-RIS systems for next-generation networks.