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This paper proposes a novel technique and algorithm for the simultaneous sensing of three-phase currents based on multiple air-core coils. The basic principle for the current measurement in one conductor using one and two air-core coils is derived, and a means of self-calibration is determined. In order to measure the three-phase currents in parallel overhead lines, a novel smart current sensor has been developed, consisting of four air-core coils located in close proximity to the conductors. Algorithms are derived, simulations and extensive experimental verification is conducted, and a first prototype is fabricated and tested. Measurement errors introduced by variations in sensor orientation and the effect of adjacent conductors are both analyzed. The proposed technique and algorithm promise a novel smart current sensor for three-phase currents, simultaneously sensing the overhead conductors, which are low cost, small in size, easy to deploy extensively, do not require clamping around the utility conductor, and have the capability to self-calibrate. Integrated with the wireless-sensor network technique, these smart sensors can be widely applied in dynamic state estimation, monitoring, and optimal operation of the distribution network, and will become an integral part of energy-management systems of future power distribution networks.