Abstract: Regarding hydrokinetic energy harvesting and efficiency optimization of vortex-induced vibration (VIV) in rigidly coupled three-cylinder oscillator systems, the vortex-induced vibration response characteristics of triangularly arranged three-cylinder oscillators were systematically investigated using two-way fluid-structure interaction numerical simulations. The analysis focused on single spacing ratios (G=0.5,0.2,0.19,0.18) and composite spacing ratios (G₁=0.5 with G₂=0.2/0.19/0.18), examining the effects of reduced velocity (U_r=2～13) on amplitude, frequency, lift coefficients, wake patterns, and energy conversion performance. Key findings include: The single small spacing ratio (G=0.18) achieved a peak efficiency of 50% in the lock-in region, while composite spacing configurations generated higher total power (30～40 W) with slightly reduced efficiency and more complex wake evolution. Energy density for single small spacing ratios (G=0.18/0.19) exceeded 10 times that of G=0.5 and doubled that of G=0.2, with composite spacing arrangements exhibiting intermediate energy density levels. The findings provide a theoretical basis for optimizing spacing ratios and configurations in multi-cylinder energy harvesters for low-velocity hydrokinetic energy extraction.