Universal high-power three-phase mains interfaces for electric vehicle (EV) charging must provide a wide output voltage range (e.g., 200V to 800V) and thus provide buck and boost capability. An advantageous realization combining a three-level (3-L) T-type (Vienna) boost-type PFC voltage-source rectifier (VSR) with a 3-L buck-type DC/DC converter stage is presented in this paper. For high output voltages (boost-mode), the VSR-stage operates with 3/3-PWM, i.e., continuous PWM of all three phases to regulate the output voltage while the DC/DC-stage remains clamped to avoid switching losses. For low output voltages (buck-mode), the DC/DC-stage advantageously controls the DC-link voltage according to a time-varying reference value, which allows to sinusoidally shape the currents of two mains phases, such that the VSR-stage can operate with 1/3-PWM (only one of the three bridge-legs operates with PWM at any given time) with reduced switching losses. This paper proposes a novel 2/3-PWM scheme for the output voltage transition region, where output voltages are between the buck-mode and the boost-mode. This enables loss-optimum operation (i.e., the minimum number of the VSR-stage bridge-legs operating with PWM, and with the minimum possible DC-link voltage) for any output voltage. Furthermore, this paper introduces a new synergetic control concept that ensures seamless transitions between the loss-optimum operating modes. A comprehensive experimental verification, including pre-compliance EMI measurements, using a 10-kW hardware demonstrator with a power density of 5.4kW/L, a peak efficiency of 98.8% at rated power and 560V output voltage, and >98% efficiency for all operating points with >400V output voltage and more than about 50% of rated power, confirms the theoretical analyses.