This work proposes a piezoelectric converter for energy harvesting composed of a rigid ball enclosed among piezoelectric diaphragms arranged in a three-dimensional geometry. When the structure is excited by mechanical vibrations, the ball repeatedly bounces and hits one or more diaphragms, implementing the impact technique in a multi-degree-of-freedom configuration. The converter was designed and built and a single-axis two-diaphragm version was characterized under sinusoidal and random vibrations from 10 Hz to 100 Hz at different amplitude values up to 1.8 gRMS. The experimental results show that the maximum RMS converted power provided by each diaphragm both with sinusoidal and random excitations is about 4 μW. The triaxial converter was tied to the ankle of a person and, in about one minute of walking at 4 km/h, 1 mJ of energy is extracted and stored by a custom rectifier circuit into a 470-μF capacitor.