A three-freedom-degree transmission mechanism comprises a helical pair, a revolute pair and a cylindrical pair. The axis of the helical pair is perpendicular to the axis of the revolute pair, and the axis of the revolute pair is parallel to the axis of the cylindrical pair. The revolute pair comprises a support (7), a movable sliding block (5), the helical pair (3), a nut (4), a left limiting block (1) and a right limiting block (6). The left limiting block (1) and the right limiting block (6) are fixedly arranged on the support (7), and the helical pair (3) is provided with the movable sliding block (5). The transmission mechanism is characterized in that the helical pair (3) mounted on the support (7) and the revolute pair can form 0-degree to 180-degree intersection angles through the movable sliding block (5). According to the transmission mechanism, under the condition of identical number of rotation of a screw rod, the speed of the straight line motion (opening and closing) of the nut (a door leaf) is improved. The mechanical locking of a vehicle door is achieved without another set of mechanical locking mechanism. The transmission mechanism is simple, the motion procedures of unlocking and locking are removed, procedure control is simplified, and two fault hazards are eliminated.