As quantum key distribution progresses into commercial products and services, there is growing interest in technological solutions that improve performance and/or reduce costs. We present an experimental demonstration of the polarization-based three-level decoy-state BB84 quantum key distribution (QKD) protocol, at a clock frequency of 50 MHz, that relies on fewer resources at the receiver and increases robustness to side-channel attacks for both the transmitter and receiver. A single-laser source and cascade of electro-optic modulators provide the four polarization states, while a photonic recombiner with time-multiplexed inputs enables detection with a single detector.