The trivacancy (V3) in silicon has been recently shown to be a bistable center in the neutral charge state, with a fourfold-coordinated configuration, V3[FFC], lower in energy than the (110) planar one [ V. P. Markevich et al. Phys. Rev. B 80 235207 (2009)]. Transformations of the V3 defect between different configurations, its diffusion, and disappearance upon isochronal and isothermal annealing of electron-irradiated Si:O crystals are reported from joint deep level transient spectroscopy measurements and first-principles density-functional calculations. Activation energies and respective mechanisms for V3 transformation from the (110) planar configuration to the fourfold-coordinated structure have been determined. The annealing studies demonstrate that V3 is mobile in Si at T>200 ∘C and in oxygen-rich material can be trapped by interstitial oxygen atoms so resulting in the appearance of V3O complexes. The calculations suggest that V3 motion takes place via consecutive FFC/planar transformation steps. The activation energy for the long-range diffusion of the V3 center has been derived and agrees with atomic motion barrier from the calculations.