A fast and high-efficient sensitivity-based approach for real-time three-phase line flow calculation is proposed in this paper, this approach combines the three-phase power-flow (TPPF) solution techniques with the new sensitivity factor, named three-phase ZBUS distribution factor (TPZBD). The TPZBD is associated with both the changes of real and reactive power injections of individual phase at each bus into the three-phase line flow. Hence, the proposed TPZBD directly reflects load demand changes to the line flow. Accordingly, any feasible TPPF solution technique must be used to solve for the base case line flow solution at first time, based on the solution and TPZBD formulas, the real-time three-phase line flow is determined rapidly by the presented approach without any iteration after load demand changes. Finally, a 7-node sample system that is modified from Taipower distribution feeders is used to test the proposed approach. The results of this paper demonstrate that the proposed approach is capable of calculating real-time three-phase line flow for distribution automation.