In response to the current situation where the current measurement of three core cables directly uses sensing signals to measure the position and current of multiple conductors, the number of sensors is uncertain and the number of sensors used is large. This not only makes the multi-conductor current measurement system lack a determined sensor usage strategy but also leads to large measurement errors. This article analyzes and provides the minimum and optimal number of magnetic sensors required for the non-invasive measurement of three-core cable current. The principle of non-invasive multi-conductor current measurement is proposed. Based on the magnetic field distribution relationship of the three-core cable current, 6 magnetic sensors are used to surround the cable distribution. The optimal estimation of 2 eigenvalues is obtained from each sensing signal, and 12 independent equations are established to achieve the detection of conductor position and measurement of conductor current in the cable. A three-core cable current measurement platform was built and tested for verification. The test results show that the maximum amplitude error of the method for measuring three-phase currents ranging from 15 to 100 A is less than 1. 00% , the phase angle error is less than 2°, and the conductor position error is less than 0. 2 mm. The proposed measurement principle greatly reduces the online measurement error of multi-core cable current, and the measurement system has a simpler structure and accurate position and current waveform measurement results.