This study involves the design and investigation of a low-frequency, broadband, tri-hybrid energy harvester. The harvester consists of a novel septuple-stable nonlinearity-enhanced mechanical frequency up-conversion mechanism that not only enhances the output performance of the frequency up-conversion via inter-well motions, and also offers a wide and highly efficient operating bandwidth at low acceleration via the combination of resonant inter-well oscillation behavior and non-resonant behavior. The integration of an impact-driven piezoelectric generator, an electromagnetic generator, and a freestanding-mode triboelectric nanogenerator allows more energy to be harvested from a single mechanical motion, which further improves the power density. A prototype is fabricated and demonstrated using an electrodynamic shaker and various human motions. In the electrodynamic shaker test, the prototype exhibits a broad bandwidth of 2–12.5 Hz and generates an output power of 24.17 mW, corresponding to a power density of 700.3 W/m3 across a matching load resistance of 35 kΩ at a frequency of 5 Hz and 1 g acceleration. Under various basic human motions such as handshaking, walking, and slow running, the prototype can generate output powers of 38.5, 24.5, and 27.2 mW, respectively, in horizontal positions and 42.7, 10.2, and 33.1 mW, respectively, in vertical positions. A comparison study is also presented to demonstrate that the tri-hybrid prototype can produce a much higher power density than other devices reported recently. This work makes significant progress toward hybrid-energy harvesting from various human motions and its potential application in powering wearable devices.