Socially assistive robots may help the treatment of autism spectrum disorder(ASD), through games using dyadic interactions to train social skills. Existing systems are mainly based on simplified protocols which qualitatively evaluate subject performance. We propose a robotic coaching platform for training social, motor and cognitive capabilities, with two main contributions: (i) using triadic interactions(adult, robot and child), with robotic mirroring, and (ii) providing quantitative performance indicators. The key system features were accurately designed, including type of protocols, feedback systems and evaluation metrics, contemplating the requirements for applications with ASD children. We implemented two protocols, Robot-Master and Adult-Master, where children performed different gestures guided by the robot or the adult respectively, eventually receiving feedback about movement execution. In both, the robot mirrors the subject during the movement. To assess system functionalities, with a homogeneous group of subjects, tests were carried out with 28 healthy subjects; one preliminary acquisition was done with an ASD child. Data analysis was customized to design protocol-specific parameters for movement characterization. Our tests show that robotic mirroring execution depends on the complexity and standardization of movements, as well as on the robot technical features. The feedback system evaluated movement phases and successfully estimated the completion of the exercises. Future work includes improving platform flexibility and adaptability, and clinical trials with ASD children to test the impact of the robotic coach on reducing symptoms. We trust that the proposed quantitative performance indicators extend the current state-of-the-art towards clinical usage of robotic-based coaching systems.