Physically defined silicon triple quantum dots (TQDs) are fabricated on a silicon-on-insulator substrate by dry-etching. The fabrication method enables us to realize a simple structure that does not require gates to create quantum dot confinement potentials and is highly advantageous for integration. We observe the few-electron regime and resonant tunneling points in the TQDs by applying voltages to two plunger gates at a temperature of 4.2 K. Moreover, we reproduce the measured charge stability diagram by simulation with an equivalent-circuit model composed of capacitors and resistors. The equivalent-circuit simulation makes it clear that we realize three QDs in series within the nanowire, as planned. This circuit model also elucidates the mechanism of resonant tunneling and identifies a quadruple point of TQDs.