To address the problems of limited magnetic field information dimensionality, complex probe and cabling structures, and difficulty in channel expansion in existing magnetic flux leakage (MFL) testing systems, a scalable MFL testing system with parallel polling acquisition over multiple I²C buses was designed. A multi-channel probe was constructed using three-axis Hall sensors, enabling reliable long-distance communication with multiple slave devices over digital buses. An FPGA-based data forwarding module was developed to achieve parallel polling, data buffering, packetization, and real-time transmission of multi-channel data. Application tests were conducted on a PECS7-127 bridge cable specimen. The results show that the system can acquire three-axis magnetic field components from 240 channels through polling, stably obtain three-dimensional MFL information from broken-wire defect regions, and localize defects along the axial direction. This study provides a simple and easily scalable solution for acquiring three-dimensional magnetic field components in MFL testing.