The paper presents a new protection scheme for three-terminal mutually coupled double circuit transmission line using time synchronized voltage and current phasors measurements. The communication-assisted distance protection schemes are extensively used for protection of these lines. The presence of a third source-terminal causes under-reach and overreaching problems and increases the challenges during the protection of these lines. The proposed protection method uses synchronized measurements of voltage and current phasors, measured at three ends of a three-terminal transmission line. These measurements are further used to calculate voltage at the tap point. Then the superimposed voltage components are calculated at the tap point and at three terminals for both the parallel circuits. The method compares a maximum among the superimposed voltage phasor component calculated at the tap point with the maximum among the superimposed voltage phasor components measured at the three terminals of the line to identify the faulty line. A 400-kV three-terminal mutually coupled double-circuit test system is simulated in ATP/EMTP environment and the scheme is verified in MATLAB. The simulation results confirm the superiority of proposed scheme.