Based on the finite-volume-method (FVM), the vortex-induced vibration (VIV) of three tandem circular cylinders is numerically simulated using the open source OpenFOAM. The fluid-structure interaction and dynamic response characteristics are analyzed with Reynolds number Re
= 150 in the spacing ratio range of 2 ~ 6 and reduced velocity range of 2 ~ 16. The results show that, when the spacing ratio is 2, the lock-in region of the upstream cylinder vibration becomes significantly wider than those at other spacing ratios, and the amplitude of the upstream cylinder in the lock-in region is significantly higher than the larger spacing ratio cases. It indicates that the vibration of the upstream cylinder is enhanced at small spacing-ratio arrangement. Due to the effect of wake-induced vibration, the maximum amplitude of the middle cylinder and the downstream cylinder are higher than that of the upstream cylinder. Five wake interference modes are identified, including the overshoot mode (OS), continuous reattachment mode (CR), alternate reattachment mode (AR), quasi-co-shedding mode (QCS) and co-shedding mode (CS). Nevertheless, due to the dynamic alteration of amplitude during the vortex-induced vibration process, the wake interference mode undergoes unstable switching. When the spacing ratio is 2 with the reduced velocity of 7, a "beating" phenomenon is observed in the response. When the spacing ratio is 6 with the reduced velocity of 4, a large periodic phenomenon is observed due to multiple frequencies participating in the vibration. Both the two phenomena cause the time-varying lift coefficient and response amplitudes of the cylinders. When the wake interference mode between the middle cylinder and the downstream cylinder presents the quasi-co-shedding pattern, the two-layered vortices in the wake merge to form the secondary vortex street. Such a merging occurs among two or three adjacent vortices on the same side, which is related to the spacing ratio of the cylinders.